Microsoft accepted some paid advertising for banner ads on Windows Live Messenger and MSN that inflicted some harm/pain on some users. This malware is called System Doctor 2006 which inflicts Winfixer and Errorsafe. Sandi’s Spyware Sucks site has the details.
My ad blocker of choice is Ad Muncher. It even works on x64 Vista. It kills ads on Windows Live Messenger and IE.