Searching for ubuntuforums in google, http://www.google.com/search?num=20&q=site%3Aubuntuforums.org, google responds with 403 forbidden and a warning about viruses!?!?
However by removing the num=20 (how many links per page) the query works fine. Looks like a caching problem by google.
How odd.
It did get me concerned for a few secs, then I remembered Im not using Windows.
Someone on your subnet or rather several computers on your subnet have been infected by a worm. Google protects itself against dDos attacks and what not by banning searches from an entire subnet for a short period of time. I believe you can find out more info on the help pages at google.
Its not implying that you have a virus, its just someone/or several people on your subnet has one. :)