I'm probably naive - but what percentage of XP installations in Derbyshire are likely to be dodgy? I *think* the majority of users probably buy their computers ready-made with the OS pre-installed and then never think about it again. If you do run across a machine with a pirated OS, tell the customer that you can fix the computer but as it's an illegal copy of the OS you won't be able to reinstall it if you have to wipe the harddrive for any reason. If there is illegal software loaded that you concude is causing a problem (virus, adware, spyware, malware, etc.) tell the customer that it's the cause and that to solve the problem it must be removed, but that you can't provide a "clean" copy so the decision is their's to take. You could have an agreement that the customer signs up front acknoweldeging all of this up front. Actually, you'll need an agreement anyway that covers you against loss of data and hardware failure, etc. - you could just incorporate it.
You should/could also ask that customers provide you with the media for their OS... I *guess* if they provide it to you and it's a pirated copy, it's up to you whether you hold your nose and load it or not. I'm talking in circles this morning - late night and early morning - not pretty!