Some malware infections are targetted at blocking access to the Registry Editor by changing a value in in the Registry itself, and even when the infection has been removed, the Registry remains blocked because your antivirus program has no way of knowing what the original value was.
There's a freeware utility here that allows you to block/unblock it:
http://www.xeromag.com/fvshare.html
on the page, scroll down to "REGISTRY PERMISSION TOOL". Download the zip file. Unzip it with winzip or similar. double-click the "exe" file.
If you need an "unzipper" utility like Winzip to unzip it, you can get it here:
7-Zip: (freeware)
http://www.7-zip.org/
Download the "exe" version as the "msi" installer version may cause problems on Windows 98. When downloaded, double-click to install 7-Zip.