From Suzi Turner at ZDNet blogs:
Microsoft's Windows Genuine Advantage (WGA) anti-piracy tool has angered a lot of people and rightfully so in my opinion. Fellow ZDNet bloggers have explained the details of the software along their concerns. Ed Bott wrote Microsoft presses the Stupid button and lists the stupid mistakes, starting with "it should never have been included with Critical Updates". I agree with that and all his points.
There are many more excellent links, I'll give you a hint and say that the process is called WGAtray.exe, but there are many good links, in the article, including one from My Digital Life "15 ways to disable and remove the WGA notifications." Go check it out, but first my caveat and a recommendation...
Like Jack, I've not tried it, but I have downloaded Autoruns from Sysinternals that it recommends, and I also have installed Process Analyzer from Sysinternals, which is excellent, and I have high hopes for Autoruns.
� How to disable WGA notification