If you've checked the Microsoft write ups about .Net, then you've seen that you may or may not need it, depending on the programs that you use. You do not need it just to run Hotmail or Messenger.You need the .NET framework if you wish to run software applications developed/programmed in Visual Studio.NET .All our machines, office machines especially, have .NET Framwork 1.1 SP1 installed. In the future, more and more programs will be programed which need .NET..I would leave it installed, but that's up to you. There are already some games and networking applications which require it to be installed.
If you don't need it, it can be uninstalled from "Add/Remove Programs". If you ever need it in the future, reinstall it.
Hope this helps.
Grif