Crivens said:This depends on where I got the copy of said messenger from. When it comes as part of a legitimate Windows-CD, I can buy it.
Yup.
And if you use the actual product you bought, plus its free security/whatever updates, it works
Microsoft supporting third party clients on their network = not in any way ever going to happen. If it works, it works, but Microsoft are not going to support it, and legally you've got no grounds to cry about it.