It's extremely unlikely that its a virus, as there are no Linux viruses in the wild that attack desktop Linux. There has recently been some malware designed to attack Wordpress specifically, but that's another story.
Have you installed Ubuntu or are you using it from the Live CD?
What video card is in your computer?
Many experienced Linux users do not use an AV at all; I have used AVG with no complaints (and also with no hits). Here's a good article on the Linux anti-virus scene
; it's a few years old, but still pretty much valid.