Here’s a handy list of signs your computer may be malware-infected:
System reboots, crashes, or freezes.
Slow, lagging performance.
Antivirus software suddenly not working.
Inability to install or download antivirus software or other software.
Inability to access hard drive.
Sudden inability to print documents.
Strange popup windows warning your computer is infected; pop up ads at random times; other unusual error messages.
Changes to your browser homepage without any action from you; inability to visit certain websites, or being forcefully redirected to websites you don’t want to visit.
Unresponsiveness when clicking on a desktop icon or icons missing from your desktop.
A notice from Microsoft® alerting you as to malware on your computer and asking you to pay to remove it. (Important: that notice itself is malware. Microsoft will never call you or send you an email to let you know your PC is infected.)