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When Windows Explorer is open and I click on a .bmp file, it displays a thumbnail preview on the left side of the right pane. It used to do that for .gif, .jpg, and other file types, but not anymore. I haven't been able to reassociate those files to restore the feature. What should I do?

David Turner, Little Rock, Arkansas

This Windows Explorer feature is available in Windows NT, 98, 2000, and Millennium Edition, but it works in Windows 95 only when Internet Explorer's Desktop Update feature is installed. The folder you're searching in must be set to display Web contents. In Windows 9x, click Explorer's View menu and make sure as Web Page is checked. In Windows 2000 and Me, choose Tools, Folder Options, select "Enable Web content in folders," and click OK. If an HTML template has been applied to a folder whose contents aren't displayed in the image-preview area, select the folder in the left pane, right-click an empty part of the right-hand pane, choose Customize This Folder, and use the Customize This Folder Wizard to choose a new template or to remove customizations. If the image preview area in any version of Windows is too narrow, the area may suddenly disappear. Simply drag an edge of the window to make it wide enough to accommodate the image preview.

Now you should see an image on the left side of the folder when you select a .bmp, .jpg, .gif, or other compatible file format. Unfortunately, this feature can disappear when a poorly designed installation program tampers with the Registry settings for the image files Windows is to preview. Even if you know what software caused the glitch, uninstalling the guilty party won't necessarily restore the health of Registry entries corrupted by the rogue installer. You have to do that manually--a complicated procedure.

That brings up our usual warning: Boo-boos committed while editing the Registry can cause serious problems (such as preventing Windows from starting), so proceed at your own risk. Always back up Registry files before making any changes.

In Windows 9x, make copies of the User.dat and System.dat files in the Windows folder before you do anything else. In Windows NT, choose Start, Run, type rdisk, and press Enter. In the Repair Disk Utility, click Update Repair Info and follow the on-screen prompts. In Win 2000, choose Start, Programs, Accessories, System Tools, Backup. In the Backup window, choose Tools, Create an Emergency Repair Disk. When prompted to do so, insert a floppy disk and check "Also back up the registry to the repair directory." Click OK.

Once you've backed up your Registry, you're ready to reset. Choose Start, Run, type regedit, and press Enter to launch the Registry Editor. Click the plus sign (+) next to the HKEY_CLASSES_ROOT folder icon, or double-click the icon itself. From there, navigate to the folder (called a key in Registry lingo) corresponding to the extension of the file type that Explorer is not previewing properly. For example, to fix the preview information for .jpg files, go to the .jpg key. If it has a plus sign next to it, click that to expand the tree diagram for the complete .jpg key. There you'll find a key called ShellEx. (If you don't see it, no problem: Just right-click the .jpg icon in the left pane and choose New, Key. Then type ShellEx and press Enter.)

Click the plus sign next to the ShellEx key to expand the keys underneath. There should be a key named {BB2E617C-0920-11d1-9A0B-00C04FC2D6C1}. If it's not there, take a deep breath, right-click the ShellEx key in the left pane, and choose New, Key. Then type {BB2E617C-0920-11d1-9A0B-00C04FC2D6C1} and press Enter. (You can now exhale.)

Select the key to see its contents in the right pane. It should include an icon labeled (Default) and, to the right, a series of numbers in braces. If it doesn't, and you created the key yourself, double-click the (Default) icon in the right pane to edit its value data. If the key was already there but has the wrong value, right-click in the right pane and choose New, String Value. Type (Default) and press Enter. Double-click the (Default) icon to edit its value data. Then in the Value data box, type {7376D660-C583-11d0-A3A5-00C04FD706EC} (including the braces) and click OK.

Finally, proofread your typing, both for the new key and for the string value. If you did everything right, your preview should return whenever you select a .jpg file in any Explorer or folder window that is set to view Web content.

Of course, this example fixes only the .jpg file type. You need to repeat the procedure for the .gif key and any other key that refuses to show a preview. The method outlined above won't work for every file type, but you should be able to preview images having any of the following extensions: .art, .bmp, .dib, .gif, .jfif, .jpe, .jpeg, .jpg, .png, and .wmf. Have fun!