Here are the top ten signs your PC has a virus.

How to tell if you have malware?
  1. Unexpected pop-up windows. Unexpected onscreen ads are a typical sign of a virus infection.
  2. Slow start up and slow performance.
  3. Suspicious hard drive activity.
  4. Lack of storage space.
  5. Missing files.
  6. Crashes and error messages.
  7. High network activity.
  8. Email is hijacked.
  9. Browser Woes
  10. Security Attacks

Information gather from SAGA Magazine article.