When you choose to disable Windows Messenger to start and disable run in background, Outlook XP get very slow. It seems to check messenger.
To fix this rename c:\program files\messenger folder to like c:\program files\messenger.OFF
tip from TweakXP.com
Now I can run MSN Messenger 5.0 and not run Windows Messenger 4.7 without slowing Outlook.
You can also remove Windows Messenger from winXP: type this command "RunDll32 advpack.dll,LaunchINFSection %windir%\INF\msmsgs.inf,BLC.Remove" without quotes.
Previuosly this also stopped my MSN Messenger 5.0 from working, don't know how that is with the latest build 527