If the answer to any of those previous questions is yes, then you should run System Restore immediately. If you can remember the day when you might have done something to get your computer infected, then you should restore from a point sometime before that day. If you’re lucky, whatever is infecting your machine will let you run System Restore and it’ll work fine. After you loaded the restore point, check and make sure your computer is running normally by trying to open RegEdit or any other programs that didn’t work properly before. If they are working again, you should get a virus scanner and run a full system scan, then get Windows Defender and let it run a full system scan.
One thing to remember with System Restore is that it might bring your PC back to a good working status, but the infected files are often still lingering somewhere on the hard drive. If System Restore is successful in removing the infection, you should then turn System Restore off and back on. Make darn sure your computer is in good working order before you do this. Turning it off and back on will clear out all the previous restore points so that you never accidentally restore back to the infected point.
Here’s a great article on using System Restore in Windows XP.