There are many possibilities, including malware/viruses. Assuming you've got that covered, I suggest running:
• the Deployment Image Servicing and Management tool, then the System File Checker (see https://support.microsoft.com/en-gb/...upted-system); and
• Windows 7 Disk Check (see https://support.microsoft.com/en-us/...s-in-windows-7)

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]