Have you turned off UAC completely? Are you running your account as an admin? If UAC is turned off completely AND you are running as an admin, I would suggest turning UAC back on. That way, if something attempts to reset the security settings, you'll get notified first about it. Also, not running as admin means Windows 7 will prompt you for an admin password.
This way, you can try to see if something else is turning it off.
Also, check your event logs and check to see if something is crashing causing Windows Security to turn off.
Does it say specifically what is not running in Windows Security (virus scanner, firewall, etc)?