In the UK: where should I get Windows from?

I would quite like to get Windows to run on my 2.4Ghz iMac, but to be honest I know very little about how to get hold of it. I'm in the UK.

I only want to run a couple of windows apps on it; nothing major. But I've been out of the loop regarding Windows for many years now, and don't know very much about it. Looking at online retailer (Amazon.co.uk) it seems a home edition costs £135 for Vista, or £150 for XP. I assumed that the old software (XP) would be cheaper, but it doesn't seem like it is.

There are also lots of Ebay sellers offering 'genuine' versions of XP that say they are 'from Germany' at around £35. But I don't really trust them.

So: am I missing the best way to get Windows? I'm not really a pirate-software kind of person, and am happy to pay something for it, but I thought it would be a little cheaper than it seems to be. I don't like the idea of shelling out £140 or so to Windows, but it seems there's little alternative.

So, are the ebay versions just a scam? Am I missing out on a cheaper shop elsewhere? Where should I get my Windows from?

 

OEM means I just get the disk, does it, and nothing else? Is the same legally as buying a full version in a shop?

 

Thanks for all the speedy replies!

 

One machine is fine: I'm not getting rid of my iMac for many years!

 

I was just going to go for XP - unless anyone thinks that Vista makes that much difference.