The cost of the Windows license is partially offset by the crapware bundle the manufacture infects the machine with, so you won't save much buying a blank PC and your options will be severely limited. However, you may still be able to get a refund on the license post-purchase, if you're dedicated (and lucky):

http://en.wikipedia.org/wiki/Windows_refund

Don't forget to reroute some of your winnings:

http://www.linuxfoundation.org/participate/linux-donate