I don't think you can replace the HD controller as such (but I may be wrong) so you may have to replace the mother board. Depending on the age and speed of the pooter, you might want to get a full upgrade kit with a new processor and memory too. Novatech and Maplins (search on Google for URL's) will both do that for you with a variety of prices depending on what you want. Otherwise, you could go for a new mother board - I'd try Ebay.
Hope this helps.