In my experience, and from the other sources that I've been able to gather from. The best indication of having a hostile computer virus is when you notice that your computer has been performing normal routines quite suspiciously. It may slow your system; or totally cause it to crash, you may also experience annoying abnormalities when a virus attacks basic I/O functions, such as keyboard outputs or even printers. However, if you have a decent UPDATED anti-virus software your computer should have the appropriate tools to counter most threats. Finally, I heard some rumors that some viruses or other malwares actually, or can, deceive anti-virus programs to consider hazardous materials as "friendly and trustworthy" or so they say... But, I would personally recommend that prevention is indeed the best medicine. So try everything in your power to guard that computer of yours from those wrecthed viruses and malwares. (eg. firewalls, antispy programs, antivirus...)