|The disk cleanup must have removed some necessary files. Scanreg/restore can't bring them back.|
Probably best to try an 'over the top' reinstall. That will keep your existing software and drivers intact.
Do you have the 98 installation cd? If not, boot up the computer and start tapping F8 before the OS starts to load. A boot menu should come up. Choose 'safe mode command prompt only'. At the C:\> prompt type DIR/P and enter. That will scroll through the contents of the root drive one page at a time. Look for a WIN98 directory. It it's there it will almost certainly have the installation files in it. If WIN98 isn't there they the files may be located elsewhere, or may not be on the hard drive at all. It just depends if they were copied to the drive when windows was installed. I've noticed many systems like Dell will copy the files there as a convenience should any new hardware require additional 98 files to be installed.
Check that and post back what you have.
Real men don't use AntiVirus; they just reformat