I use spam filter SpamBully for OutlookXP from http://www.spambully.com.
After Installed Service Pack 3 for Office XP I have no problems. It has always worked very well at filtering spam.

Most spam filter that are designed as Outlook plugin are more or less affected by Microsoft's update. I uses a spam filter called spamweed ( http://www.spamweed.com) which is quite unique. It also offers an outlook bar, however, the bar is merely a convenient tool. It is a pop3 (and hotmail) proxy which works with ALL pop3 email client, not only Outlook. Unique feature which I like is that 1) it uses a breadth of filtering techniques (including bayesian) which result in very high accuracy rate, and zero false positives (after a very short training period, less than one week for me. 2) It is very easy to use, and best thing is that it blocks spam before reaching my inbox, so in my Outlook I don't see a folder like "SpamWeed" or "Spam". That's very good! makes my outlook clear...

BTW, I just tried the latest v2.0 alpha, which works like a charm on my system. V2.0 significantly simplifies the setup process, user don't need to change their pop3 setting any more.