If a genuine installation of Windows 7 becomes deactivated, then it is because some system files have been corrupted. I take it that you have tried to reactivate it. You could try "sfc /scannow" run as the administrator, but if this does not work it would probably be best to reinstall the operating system. The real question is why the system files have become corrupted, this could be caused by a virus but more likely is due to memory issues. Try running memtest86 for at least three passes (preferably more) to test the memory.