Good question, ain't it?
Putting a source code under lock and key doesn't mean squat.
This is the fast way to completely remove windows messenger from Windows XP. I don't think you can just delete all the files messenger uses because in XP, it is everywhere.
Use the "cut and paste" method below, 5 seconds and its all gone (until Microsoft decides to reinstall it on you without you really knowing it when you get patches and security upgrades.)
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Removing Messenger from computers
Make sure your Internet Explorer, Outlook Express, Windows Messenger and other programs are closed before doing this.
1) Click on Start, Run
2) Type the following (or cut and paste it) into the Run line
RunDll32 advpack.dll,LaunchINFSection %windir%\INF\msmsgs.inf,BLC.Remove
3) Click on OK