You sit down at your computer and flip on the power switch, but instead of Windows' familiar sky-blue colours, you get a black screen. A knot forms in your stomach as you wonder what may have gone wrong. Maybe it's just a few corrupt files, or maybe it's ù gulp ù a bad hard disk. Either way, you pop your copy of Windows' start-up disk into the floppy drive (you did make one for emergencies, didn't you?), then reboot and use the convenient troubleshooting tools included with Windows 95 and 98 to track down and fix the problem. Right?

Not always. If the damage is serious and you can't run the full version of Windows or access the recovery utilities on your hard disk, then booting from your rescue disk can leave you stranded at the DOS prompt. You're left with a handful of DOS utilities on your emergency floppy and only a fifty-fifty chance of being able to access your CD-ROM drive. The alternative: create a better-equipped rescue disk. Here's what to do.

If you don't have one already, make a copy of Windows' start-up disk by clicking Add/Remove Programs in the Control Panel of Windows 95 or 98 and selecting the Startup Disk tab. Click the Create Disk button and follow the prompts. Booting from this rescue disk brings up a DOS prompt from which you can attempt to access your hard drive. First type c: and press <Enter>, and then run Windows by typing win at the prompt. Once launched, Win 98 includes numerous diagnostic tools for targeting problems (Win 95 does too, but not as many). If Windows won't start up, you'll have to use DOS utilities. Both Windows 95 and 98 start-up disks contain DOS versions of basic utilities like ScanDisk, Fdisk, and Format.

As you troubleshoot you may need other files, such as your system's CD-ROM drivers or Windows' configuration files. Most important are your system's DOS CD-ROM drivers. To repair damaged Windows files (or to wipe your disk clean and start over), you may need to reinstall Windows from the CD-ROM. And to read your CD-ROM drive in DOS, you must install the drive's DOS drivers. If you're using Windows 95, you'll need to have copied the drivers to your start-up disk and then referenced them in autoexec.bat and config.sys files.

Most CD-ROM drives come with a driver disk that contains the files and instructions you'll need. If you can't find a driver disk, the files may already be installed on your hard drive. In the config.sys file, you'll find the CD-ROM drivers in a line that starts "device="; in autoexec.bat, check for the line containing "mscdex.exe", the application that connects DOS to the CD-ROM drive. Those lines tell you the locations and the names of the driver files.

The Windows 98 start-up disk comes with generic CD-ROM drivers that work with most drives, but there's no guarantee that they'll work with yours. To see if your CD-ROM drive will accept the drivers, insert a CD-ROM in the drive, boot from your Windows 98 start-up disk, and type the CD-ROM drive's letter designation. Note: the start-up disk creates a ramdrive (a virtual disk in memory) with its own drive letter where it stores DOS utilities. Because the drive letter that it uses may not be the one you expect, type the dir command to display the files on the inserted CD.