You might want to try booting to "command prompt only" and running scandisk. It may not help, but scandisk occasionally does fix something. If that doesn't fix it, try running scanreg /restore (if you're on win98) at command prompt and restore a previous registry. Hopefully you'll get lucky with this one. You can try the following article on MS knowledge base
http://support.microsoft.com/support.../Q188/8/67.ASP
There's a section on doing a step-by-step and trying to get into Windows by saying yes or no to certain steps in the boot process.
Hope this helps