Hmm, I just found this on /.

"As of April 4, 2005 the SANS Internet Storm Center has raised their alert level to Yellow following a rash of active DNS poisonings. The infected DNS servers are re-directing users from popular sites such as Google or American Express to malware infecting advertising sites. According to the ISC presentation on the attack, it is believed to be linked to known spammers and malware distributors. The full presentation of information up until this point can be found here."

http://it.slashdot.org/article.pl?sid=05/04/06/170233&tid=95&tid=172&tid=218

Maybe this is your issue. I would recommend running Bind-PE (just noticed it's been renamed to TreeWalk DNS) on your local PC. I've been running it on my box for the past year and it works great. Just remember to update your root zone file about once a month.