Sounds like you may have a virus/malware. Make sure you are logged in as a computer administrator, and try again. If that doesn't work, run anti-virus and anti-malware scans with up to date definitions on both. You can get two very good tools from cNet: Emsisoft Anti-malware Free edition, and Avast! Free edition.
If the scan find nothing, you should go to Start > Computer, right-click system drive and select Properties. On the Tools tab, under Error Checking, click the button that says "Check Now..." You will be prompted to schedule a scan on the next reboot since the system drive is in use. Reboot to let the tool check for errors.
After the system boots, check to see that the problem is solved. If not, follow the instructions in the following article: