One dark day your PC freezes up, behaves erratically, or just doesn't start. Your first reaction. panic! Do you need to reinstall everything? Or maybe get a new hard disk? Or ù gulp ù buy a new PC?

Relax. Chances are good that you can fix your PC without resorting to high-risk surgery on its innards. In fact, you can carry out a number of repairs without spending a bundle on expensive calls to tech support. Here are guidelines on how to proceed.

You can spot hardware problems by closely watching the screen during boot-up. The computer exchanges greetings with each piece of hardware and confirms that it's operating properly. And it often sends a small status message to the screen. But these messages appear and disappear so quickly that they may be impossible to read.

The solution: Press <Pause> or <F8> to freeze the screen and look for error messages or other useful information. Then press any other key to continue boot-up.

If you turn on your PC and Windows 95 won't load, you may have a problem with a device like a sound card or with a corrupted software driver. To check this:

Launch in Safe Mode. As soon as you see the Starting Windows 95 message on your screen, press <F5>. This launches a bare-bones version of Windows 95 that bypasses all the start-up files and loads the plain-vanilla VGA video driver. If it loads successfully, go to the Device Manager (select StartûSettingsûControl Panel, double-click the System icon, and click the Device Manager tab). Then expand the entries in the device tree by double-clicking each one.

The tree expands to show any installed device for each type. Double-click a device's entry to open its Properties box, check the Device Status area for conflicts, and then click the Resources tab and look at the Conflicting device list. If it reports a conflict, launch Windows 95's Hardware Conflict Troubleshooter (select StartûHelp, click the Contents tab if it's not already on top, and double-click the Troubleshooting icon to open the list of Troubleshooters). If there's no conflict, shut down and try this:

Launch with step-by-step confirmation. After you spot the Starting Windows 95 message, press <F8>. This will display the Windows 95 start-up menu. Option 5 walks you through start-up one step at a time. You can load or skip each device listed in the Registry and each action specified in your autoexec.bat and config.sys files. An error message will appear for any device or program that fails to load properly.

When you start step-by-step mode, you'll have the option to create a start-up log file, called bootlog.txt, in the root directory. Do so. This documents the successful ù or unsuccessful ù completion of each step. The data is arcane, but it can save you lots of time if you end up calling technical support.

If your PC loads Windows 95 but has other problems while you're working, check Device Manager for hardware conflicts. If you have a good diagnostics utility handy, now's the time to use it. Note that utilities such as Norton WinDoctor work best when they're installed before problems occur.

Caption: Norton's WinDoctor identifies and labels file problems by their severity

If you don't have a diagnostics utility on hand and need a solution now, start here:

Check your hard disk. Detect file errors and physical damage by running ScanDisk (ProgramsûAccessoriesûSystem ToolsûScanDisk). Select Thorough under Type of test. If your disk has substantial physical damage, you want expert help. If it's got only a few damaged sectors, ScanDisk marks them so they won't be used, and tries to salvage their data. Run ScanDisk often; if new bad sectors keep showing up, your hard disk may be failing.

A few cross-linked files or lost sectors are nothing to worry about. But if ScanDisk reports lots of them ù and especially if their filenames have turned to gibberish ù the File Allocation Table may be damaged. This is serious. You may need to reformat your hard drive.

Run an antivirus program. Viruses are easy to acquire, especially if you download lots of material from the Internet. Fight back with a good antivirus program.

But something as simple as shutting down your computer without closing Windows 95 properly, or deleting unwanted software instead of uninstalling it, also can lead to glitches and annoyances. Windows 95 and application programs operate in a complex web that relies on individual files located all over your hard disk. Consequently, a single corrupted file can get you knee-deep in trouble.

Hunting down the file at fault is like searching for a lost penny in a bog. Your best bet is to reinstall key files while changing your hardware and software settings as little as possible. The first step:

Reinstall suspect applications. If you're having trouble with a single program, try uninstalling and then reinstalling it. Don't just manually delete the program's files, because this may leave orphaned Registry entries and other files. Many applications come with an uninstall program that can be launched from the StartûPrograms menu or from the Control Panel's Add/Remove applet. If you can't find an uninstall program, try a dedicated uninstallation utility. The next step:

Verify and reinstall Windows 95 files. To verify that no Windows 95 files are missing or corrupted, run the setup program from the Windows 95 CD or installation disk. The program will recognise that Windows 95 is already installed and, if you have the upgrade release of Windows 95, will display a dialogue box with two choices. Select Restore Windows files that are missing or corrupted and proceed with the setup. If you have the standard version of Windows 95 ù the one that came with the PC, for instance ù just perform a standard reinstallation. Either way, it will replace missing or damaged files. If Windows 95 asks whether you really want to overwrite a given file with an older version, make your best guess. You probably do, but this is not always true.

Remember to close down all programs before you reinstall Windows 95, including any that automatically run on start-up.

Replace Windows 95 Registry files. Windows 95 continually tracks and stores your PC's current hardware and software configuration in the Registry. A sudden shutdown can leave Registry files incomplete or even damaged, leading to strange behaviour.

Windows 95 stores Registry information in the files user.dat and system.dat in the Windows folder (in DOS, type attrib user.dat -r -s -h, for example, to remove read-only/system/hidden status for these files). Fixing a damaged Registry file is easy. Just overwrite the file with an undamaged copy ù if you can find an undamaged copy, that is.

Every time Windows 95 launches successfully, it does automatically make backup copies of Registry files, called user.da0 and system.da0. But if your files are already damaged from a previous session ù say, after a power failure ù then these backup copies will be damaged, too.

So you should make backup copies of user.dat and system.dat, and update them frequently ù at least once a month ù as well as every time you make a hardware or software change to your system. Because you may not discover a problem immediately, it's best to keep a chronological series of backup copies.

Don't use the utilities on the Windows 95 CD to back up the Registry, since they're buggy. Instead, just make a copy of each file and rename it. Ideally, you should make two copies, one on your hard disk and one on a floppy.

If you don't have a good recent backup copy of the Registry, there is one last hope. Windows 95 makes a copy of system.dat when it finishes its initial installation. The file is called system.1st and is located in the root folder. Like system.dat, system.1st reflects your PC's hardware configuration at the time of the initial Windows 95 installation, so you'll have to reinstall any hardware you added subsequently. If, however, your problems stem from user.dat ù which focuses on software configuration ù you're out of luck.

Nothing worked? Sometimes you just need a fresh start, especially if you've installed and uninstalled lots of programs. If you do decide to reformat your hard drive and reinstall everything, start by saving key Windows 95 files for future reference.

Caption: If you're troubleshooting PC problems, backup copies of these files can be lifesavers. Autoexec.bat, config.sys and system.1st are located in your PC's root folder; the other files are in the Windows 95 folder (typically c:\windows)