Saab is right,
You cant tell if you have or have not had a virus - I think the correct phrase would be to say you never noticed a virus.
Not all of them are malicious and alot are very cunning and stay hidden gathering information or performing functions for other ppl remotely. Or stay dormant until a specific event occurs (e.g. 1st april).
Not every virus is picked up by the virus checkers, alot of virus checkers only work on the principle that they look for the virus they can identify - so if your anti-virus software provider has not seen the virus before it will not be in your updates to prevent.
The better virus checkers will store and refer to file sizes and checksums so that it can identify that even if it does not know if a virus is present or what it is - it does know something has been changed that should not have been.
Please dont get the false sense of security that because you have a virus checker/blocker all will be okay - virus protection is still very much in the realm of a reaction to a problem rather than a prevention.
Cheers