It's not an ad, I think. THis happened to me on another thread back last year, probably September or October (or August?). Someone's signature was hosted by a site that matched the url displayed in the "Malware Detected" page.
The best way to do it is to use an Internet Browser other than Google Chrome to find any images, avatars, or signatures that match up with the "Malware source URL". If none of them matches, then it's probably an ad (though I don't think this site would connect to "animedownloads.us" for ads).
Edit:
Found it. It's Vitor's images in his post and all posts that quote from his. They come from the domain "animedownloads.us".