This offers been shown on psy-forum before and the conclusion we came to was don't bother.
Read all the small print. The computer is never actually yours (so you can't flog it off). And the advertising will really get on ya tits after just a few hours (You have you answer questions about the adverts shown).
If you don't mind then go for it. Just make sure you know all the facts before diving in.
If you have another shite PC you could pull the hard drive out the new PC, stick it in the shite pc and let the shite pc go on advertising to itself while you go about your business on your clean new PC. You can get a new 40Gb HD for next to nothing.