You should contact the manufacturer of the computer (not the reseller and not Microsoft).
It's unlikely they'll just refund your money. You'll need to insist, and be prepared to stand your ground - they will evade, and invent excuses.
Record all communication: phone calls ('This call may be recorded...' does not state which party may record the call, so phone conversation is arguably fair game!), letters, screenshots to demonstrate that you've removed Windows, etc.
The legal background is this:the manufacturer is bundling their hardware with a third party product ('tying'). In the UK (and EU) this violates the 1998 Competition Act, and gives you grounds to take the manufacturer to a Small Claims Court.
There's a whole section in the Civil Procedure Rules devoted to competition issues: http://www.justice.gov.uk/civil/procrules_fin/contents/pract...
Basically, you will need to inform the Office of Fair Trading about your case, and apply to transfer the hearing to the from the usual County Court to the Chancery Division. A county court is obligated to help with procedure (but never advice), so don't be afraid to embark on such action yourself.
I'm involved in a similar action at the moment against Samsung. Unfortunately, the Office of Fair Trading brushed aside my case, so I've made a formal complaint against THEM. I'm pushing ahead carefully and intend to see this through to the end.
Of course, these laws and procedures apply in the UK (and similarly throughout the EU), but if you're in the US or elsewhere, there ought to be equivalents.
The most important thing to realise is this: you DO NOT need expensive legal representation, and the anti-competition laws are designed to protect consumers from these practices. It may be that the manufacturer will surrender to your demands if they see you're serious.