To use System Restore, navigate to...
Start > Programs > Accessories > System Tools > System Restore
Choose "Restore my computer to an earlier time" and click on Next. Then choose a date from before this problem started. And no, you don't have to backup your file. You may, however, have to reinstall any programs that were installed after that date. If System Restore doesn't fix your problem, you can undo the process.
Did you try deleting the user accounts you created?