I tried disabling it through gpedit.msc, (Start/Run/gpedit.msc/Admistrative Templates/Windows Components/Windows Messenger) but that doesn't work. Although I specifically stated that it should NOT start at windows boot, it still does.

Even uninstalling it doesn't work !!! The program is gone, but other people can still see me online and page me. Incredible.

For now I've solved it like so : I've put it into Zonealarms' forbidden programs list. That will stop it for now, but is far from an ideal situation.

Edit. Just after posting this post, my PC froze. It seems that this msmsgs.exe constatly tries to get online. If the firewall blocks this the PC crashes.
The weird part is (is this can even get more weird) that once I kill this process the first time, I remain offline, even if the process revitalises itself after a few minutes.

The way I see it now is that I'll just let it boot with windows, like it wants to obviously. Then I'll exit the program manually each time. A drag, but I can't think of anything better.

Second edit : Ok, that too doesn't work. WhenI immediately close messenger after, I remain online. AAAARGH!

Any other ideas are highly appreciated !!