Please provide more information: part of your post seems to have been cut off.
What did you do or try to do with the Windows 98 disk - especially where you have XP?
What is the basis for suspecting a virus/worm? Did you AV software detect it and attempt to clean.
If you can boot and are in Safe Mode then you should be able to look at the Event Viewer logs and see what error(s) are being logged. Need some error code or warning message.
Probably some process or service has been corrupted - system then defaults to safe mode.