A lesson to learn is to, always no matter what patch up all your computer software including the OS. Most virus nowadays infect systems, by exploiting security holes, so if you system is fully patched up, your changes of getting infected is pretty slim. Of course, if a virus has been written to exploit a zero day vulnerability, you are pretty much screwed. However there are software that can temporary close those security holes, until the vendor has released a patch. I know McAfee has a product that does that, but I forgot whats called.
Edit: Anyway, I would recommend using Avast5. Avast never let down.