spam messages

Ever since I got a computer with XP, I've been getting
these unwanted spam messages on Windows Messenger. About
95 percent of them are messages urging me to buy some
program to stop getting these unwanted messages.

Is there any way to TURN OFF Windows messenger? I never
knowingly signed on. I do not use MNS or whatever the
Microsoft network is called.

I'd really like to stop these annoying messages which
keep telling me the can stop thsee annoying messages.

Any help would be appreciated.