My system has been very unstable. An up to date, full featured antivirus solution should give you mostly accurate information on your system. Knowing the anti-virus product actions, and familiarity with your system, will tip you off to possible problems or false positives from the virus scanner. A viruses' primary agenda is to infect the system and replicate itself, so it will probably conceal itself as much as possible at the time of infection, and possibly for some time after. Some viruses indicate their presence by musical, graphical or text displays.
Some applications can exhibit behavior particular to infection.
Many viruses will announce their presence in very subtle ways. Most common system problems stem from other sources, like extension conflicts, file corruption etc. New versions of virus software interacting with system software or an application have been known to cause malfunctions. Checking the manufacturer's online support area may clarify some known problems. Employing antivirus software and using it regularly will help you rule out virus infection rapidly.