This "messenger service" has nothing to do with anything like Instant Messenger or other chat methods.

It was originally intended for network administrators to use to communicate to client machines on the network.

http://en.wikipedia.org/wiki/Windows_Messenger_service

Unfortunately, it is extremely vulnerable to hacking and other exploits. Basically, a hacker could scan your IP address, find the messenger service ports active, and hack in through the service.

If the service is running, you are "using it", meaning it is active in the background. Disabling the service is very unlikely to affect anything on your system. I've never had a problem because of it being off. But it absolutely increases your security with it disabled.

After XP SP2 it should be off by default, but make sure it IS disabled.

Brian