Jay said:

Since AVG's site doesn't seem to define what the threat actually is, it's hard to tell if it's a problem or not. Is it a virus, or is it just a warning against blackhat SEO techniques (which I don't knowingly use and aren't a real threat of any kind regardless)?

It's a warning that someone else is using blackhat SEO techniques by, presumably, exploiting a vulnerability on ot.com's server to inject those backlinks. Although I don't know what they expect to gain from it as I'm pretty sure Google's algorithms are clever enough to ignore non-displaying links.

I honestly don't think there's anything to worry about, but that's easy for me to say as a Mac user ;)

I also don't think it's anything for end users to worry about, but you should contact your hoster about it.