The first thing to rule out is a virus. Download TDSSKiller and RKill from our
Freeware DB. If you do not have an anti-virus then download, install, and allow to update Malwarebytes Anti-Malware (MBAM). Then reboot your system into
safe mode without networking (Reboot and, when it says "Starting Windows," Press F8; If you get a message asking continue in safe mode or perform a system restore, choose continue in safe mode). Now run TDSSKiller and after that run RKill and after that run a full virus scan. The first two will take maybe 5 minutes each but a full virus scan can take anywhere from 30 minutes to several hours depending on your hard drive size and number of files.
Booting in safe mode makes sure only minimal services/programs are started. TDSSKiller is designed specifically to look for rootkits. RKill is designed to stop the stealthing programs that hide/protect/regenerate some nasty viruses. Running these first will help ensure your A/V has the absolute best chance of cleaning up infected files.
If you do not have a virus then my next guess would be a corrupt registry and also the possibility that your hard drive is going bad. Try a System Restore to a Restore Point prior to the problems. And also scan the hard drive for problems: open windows explorer and right-click on C: , choose properties, Tools tab, click check now, in the new window make sure "scan for and attempt recovery of bad sectors" IS checked, select Start. It will then want to reboot and run the scan.