Depending on what you want to run, you could do what a friend of mine does. He switched to Linux in 2007 when he saw where Microsoft was taking Windows with Vista. He still has to work with Windows as part of his job so he got VMWare for Linux and runs Windows XP on a virtual machine. It's a lot cheaper than a complete machine.
The only thing you can't do with that setup is play games, for that you'll need a real machine running Windows.