Windows Messenger (formerly MSN Messenger), the internet chat program that comes built-in to Windows, is application that many of us refer to as a "virus."
(Note that it is not actually a virus. It received that name because of the following virus-like activity):
1) It will often start itself up at seemingly random times
2) The user cannot stop it from starting up at random times (until now :o)
3) The user cannot even shut it down without terminating the process in the Task Manager
Here's how to stop Windows Messenger from taking over your computer:
Go to the Start Menu --> Run and type in gpedit.msc
When the Group Policy windows comes up, under Computer Configuration click Administrative Templates.
Then, on the right sight of the window double-click Windows Components and then similarly Windows Messenger.
Still on the right side of the window, double-click Do not allow Windows Messenger to be run.
When the dialog comes up, click Enable. Now it should no longer run on your computer.
Note: Keep in mind that if you ever want to use Windows Messenger for any reason, you'll need to follow the same steps above, only click Not Configured instead of Enabled.
As a side note, you can similarly stop Windows from automatically trying to run every single "disk" (CD-ROM, USB Flash drive, digital camera, etc.) that is ever inserted into the computer. The sequence is as follows:
Computer Configuration --> Administrative Templates --> System --> Turn off Autoplay --> Enabled
Good luck !