The iframe is not a virus, from the Mudcat user's viewpoint. What it does is entirely normal web coding - it just has no conceivable innocent purpose. There's no reason for any anti-virus program to flag it. If you aren't seeing it there's something wrong with your browser, since it should be loading iframes when it sees them.

But it seems likely it was some sort of virus or trojan that put it there. Only someone with access to Max's hardware can figure out what happened and fix it.