Can Microsoft legally block Windows XP activations?

One of the new features introduced in Windows XP is that you need to activate it online if you’re going to keep using it beyond a certain point. If Microsoft decides that Vista isn’t being adopted as widely or as quickly as they like, can they turn off the XP activation server and force their customers to upgrade?

From here :

I imagine if Microsoft didn’t follow through on this, they’d have a lot of civil lawsuits on their hands …