Test for Viruses and Spyware

When everything else seems okay, but you continue to see some kind of evidence that the computer is not working properly, it's possible that it has been infected by one or more viruses or invaded by spyware. So it's always appropriate to perform complete system scans to either find and remove the problem or eliminate those problems from your list of possible causes.

CROSS-REF 

Chapter 48 provides information about spyware and viruses, as well as pointers to antivirus and anti-spyware programs.