False Positives happen. Just at work the other day I had NAV Corporate (older version, latest definitions) jump up and down about a trojan in a Forum I was browsing, now I admin this forum, and it has no ads. I saved the HTML locally, and NAV jumped up and down about it there too. I opened it and looked at all the code, and there was NOTHING out of the ordinary about it, nor did NAV 2003 at home think it anything out of the ordinary. NAV 7 jumped up and down, NAV 2003 said all was good.
It's a flaw of virus scanners, some viruses can be tricky and change, so they write virus scanners that look for 'similar' peices of code, even if they don't do anything mallicious, or really anything at all. The other end of it is though, if you did get a virus, and it wasn't picked up by your virus scanner, you'd be up in arms about how you paid money for a product that doesn't work.