As enumerated below, slow performance, unexpected actions, or blocked access to system tools may be signs of malware infection

http://www.pcmag.com/article2/0,2817,2416788,00.asp

Computers are complicated enough that they don’t always do precisely what we expect. Sometimes an unexpected behavior is just a fluke; other times, it’s an outward and visible sign of an inward and terrible malware infestation. If you notice any of these security warning signs, your system may well be compromised.

1. Popup ads appear even when no browser is open.
2. Browser navigation gets redirected.
3. A security program you never installed pops up scary warnings.
4. Posts you didn’t write appear on your social media pages.
5. A program holds your PC for ransom. Some malware programs literally hold your PC or data for ransom. Overt ransomware threats may encrypt all your pictures and documents and demand that you pay to get them back.
6. Suddenly you can’t use common system tools.  If you suddenly find that trying to use these or other system tools triggers a message saying your Administrator has disabled them, it may well be an attempt at self-defense by malware on your system.
7. Everything seems perfectly normal. That’s right. Some types of malware do their best to hide all activity, leaving no visible traces.

If you think that malware has taken up residence in your PC, install a powerful antivirus utility or security suite immediately.  Already got one? Then apparently the malware got past its protection. Make sure your antivirus is fully up to date, and run a full scan. Also get a second opinion from a free cleanup-only antivirus like Malwarebytes Anti-Malware 1.70 or Comodo Cleaning Essentials 6.  You definitely want to get that nasty, malicious program out of your system as soon as possible, before it invites “friends” to make your security problem even worse.