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
source: http://www.windowsxp.nu/
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