|If I were you I'd back up my data, and install fresh copy of windows (either from restore disk or install disk, depending what you have). Bit of a pain, and you'll need to reinstall software of course, but I suspect you have some nasty malware wrecking your windows settings (they often nobble system restore, though it does sometime just get corrupted).|
You can do a sort of 'manual' system restore if the backups are ok in System Volume Information folder. Need to boot from something like a BartPE Cd (something to give you access to the drive outside the o/s). Pick a snapshot folder, check the date/timestamps on the files, copy & rename the ones for SAM, SECURITY, SOFTWARE, SYSTEM & DEFAULT into \windows\system32\config, after first renaming the originals - in case you need them back).
This is a brief description! As I said, I'd go for rebuild.