Check for a virus.
Try running your anti virus in safe mode.
Windows 7 Safe Mode, as the computer is booting press and hold your "F8 Key" which should bring up the "Windows Advanced Options Menu. Use your arrow keys to move to "Safe Mode" and press your Enter key.
With some computers, if you press and hold a key as the computer is booting you will get a stuck key message.
If this occurs, instead of pressing and holding the "F8 key", tap the "F8 key" continuously until you get the start-up menu.
Once you're done in Safe mode if you want to get back into Normal Windows restart the computer like you normally would and let it boot normally.