Maybe it got corrupted somehow, or sometimes the .net site is totally fuk'ed. Does happen occassionally.
If you're still having issues I'd simply reinstall messenger. There is a new messenger with SP1, which is out tomorrow or you can get it direct from MS now,<A HREF="http://download.microsoft.com/download/whistler/SP/SP1/WXP/en-us/xpsp1_en_x86.exe" target="_new"> here</A>. Also you can try to reinstall the messenger client, but I cannot find it for download alone anywhere. Try windows update for an update for it, or if need by uninstall it manually and reinstall. To do this follow the directions below:
1. To remove messenger go to START > RUN
2. Paste the command in #3 and hit "OK"
3. RunDll32 advpack.dll,LaunchINFSection %windir%\INF\msmsgs.inf,BLC.Remove
4. Reboot and now reinstall messenger by going to control panel > add/remove programs > window components and check windows messenger, hit ok and it will reinstall.