Q How do you break the association between a specific file type and an application? I recently double-clicked an .asu file, and Windows asked me what program to open it with. I clicked Internet Explorer, only to discover, first, that IE can't even open .asu files and, second, that Always use this program to open this file was checked. So now double-clicking an .asu file launches IE, which doesn't do any good. How can I reset Windows so that there's no association for .asu files?

A Microsoft obviously never imagined that you might want to dissociate a file extension from an application. To complicate matters, installation programs reassociate extensions all the time (since I install a lot of utilities, I never know what to expect when I double-click a .zip file).

There are two ways to ensure that an extension won't be associated with an application. The easier way usually works, but I'll tell you both.

In Windows Explorer, select ViewûOptions. In the Options dialogue box, click the File Types tab. Look for the appropriate file type, which will probably be the extension name followed by the word file ù for instance, ASU File. If you don't find it here, look under the application name.

Once you've chosen the file type, scan the bottom of the dialogue box, where the word Extension appears. If the extension you want to dissociate is the only one listed, you're in luck ù you can use the easy way.

You may be asking, "Why not just select the file, press <Shift> while right-clicking it, and select Open With to associate the file type permanently with another application?" Here's why: if you don't investigate the Options dialogue box, you won't know what other extensions are appended to this file type. And you may still want .htm files to open with IE.

Caption: Dissociating an extension from an application is easy if Windows identifies that extension as the only one in its file type. But if it's one of several, the fix is trickier

The easier way. In the Options dialogue box, if the errant extension is the only one associated with the file type, while you have the type selected, click Remove, Yes, then Close.

The harder way. If the extension is one of several connected with the file type, and you want to remove only one, you'll have to edit the Windows Registry. This is an inherently dangerous task, so make sure that you back up your Registry files (user.dat and system.dat) before undertaking it. Once you've made your backup, select StartûRun, and enter regedit to open the Registry Editor. In the left pane, expand HKEY_CLASSES_ ROOT and look for a folder with the name of the extension you want to remove. Delete that folder, close the Editor, and reboot.