Well the cheapest way I guess is if you are able to get it via an educational licence. If money is the issue though I'd probably hang out and grab Windows 8 when it is released at the early adopter discount.
My XP crashed horribly a week or two ago and so I thought I'd give the 8 evaluation a go again. Metro still sucks on the desktop, takes a few days to work out where some things are (and Metro isn't really that intuitive), however you soon learn just to ignore it and treat it as a large start menu. Have to say that the last couple of updates have actually made it stable on my Z68 system. For $40 or whatever it is a reasonable upgrade cost from XP ($100 and I'd say no chance!)
I would rather have Windows 7 too, but looks like 8 is the affordable way to update from Vista/XP.
Edit...and oh...the weather button on Metro is quite nice lol