Sorry for late reply, been busy.
In these days most multiplayer games have online accounts you have to access before you can play the game. In this case you will have to make sure your account data is anonymous as well as your IP when you play games.
The easiest way I can come up with on the top of my head is using a VPN. Windows will simply tunnel all your traffic to the VPN. And yes, you can change your GUID in registery, but I wouldent recommend it.