Windows 2000

Click Start->Programs->Administrative Tools->Services 
Scroll down and highlight "Messenger" 
Right-click the highlighted line and choose Properties. 
Click the STOP button. 
Select Disable or Manual in the Startup Type scroll bar 
Click OK

Windows XP

Click Start->Control Panel 
Click Performance and Maintenance 
Click Administrative Tools 
Double click Services 
Scroll down and highlight "Messenger" 
Right-click the highlighted line and choose Properties. 
Click the STOP button. 
Select Disable or Manual in the Startup Type scroll bar 
Click OK