If you are not using it for gaming you should be able to get a replacement quite cheaply. My suggestion would be to go to a local computer shop, explain what you need and have them build one for you.
Or... do you not have a mate who's a bit of a computery geek that can take some of the old components (i.e. graphics card, hard drives, sound card, case, fans, etc) and simply replace what has died on your old system? For instance you may able to replace the motherboard, processor and RAM for as little as 150 quid but if you go for a whole new box you could be talking 250 - 400 quid.
Having said all of that something like this would probably do for you -->
http://www.aria.co.uk/Systems/Home+...ual-Core+Next+Day+Desktop+PC+?productId=51511
Or this (at the most IMHO seeing as you dont need a gaming machine) -->
http://www.aria.co.uk/Systems/Home+...ual-Core+Next+Day+Desktop+PC+?productId=53000
I dont think they come pre-installed with Windows but if you've got the Windows disc and other software it wouldn't take much to install it on those boxes.
http://www.aria.co.uk/Systems/Home+and+Office/
Aria are pretty good suppliers, I've been buying stuff from them for years.
As always set yourself a budget and buy within that, it's very easy to be talked into over reaching yourself and buying stuff you'll never ever use to it's full capability when it comes to computer systems/components. I've known some people shell out nearly a thousand quid for a system and then do nothing more than email, surfing and the occasional letter in word! Barking mad!