Go to Start; Run; and type the command
This will run system file check, and all you have to do is follow the on screen prompts.
Have your Win 98se CD ROM handy, as if you have corrupted files, you may need to insert the CD to enable the repair.
If it doesn't work, please post back with the approximate date, when the system was working correctly, and I may be able to give you an alternative option.
Let us know how you get on please.