That’s some irritating hack!! And the links are even broken (scanner.antivir64.com/?aff=1050). The weird thing is that the url your visitors are pointed to, does not appear in the source of your website. I can’t even access the website to see what it is and a Google search doesn’t result in much. It is almost as if this is handled outside of WP. To be honest, I have no idea where to start looking, but there’s always a few things you can try of course.
1) Change to default theme and see if the problem persists. If it does not, your theme has been compromised, if it does, we have to look for something else.
2) Can you still access the WP admin? Do all internal links work or are they also redirected? What about post links (for example previews or edit links)?
Then a question: when did you upgrade to 2.6 and what plugins do you use?
[edit] another question: I see people can register. Did you get new users recently? What’s the role subscribants get? “Contributor” or something with not too much rights I hope?