I take it the hardware hasn't changed and neither has the file system, only the configuration?
In which case restore the configuration to the way it was before you introduced the problem.
The way you do this depends upon which OS you're running.
In Win98 you go to a command prompt at startup and enter SCANREG /RESTORE and select the backup automatically made by Windows on the 1st successful boot on the day the problem was introduced [or an even earlier backup].
If you did make changes to the files, then you'd need to restore a backup/image of the C: partition made when all was well. [I always keep 1 or more of those up-to-date.]