this sounds distressingly similar to a problem i have experienced when trying to run Rollercoaster Tycoon (yeah, the first one) on windows XP, both here at home and on XP machines at work.
the only problem is, all the info i've seen about the RCT problem is that it's to do with RCT being older than windows xp and therefore having compatibility problems with XP. The only RCT patches available now do not work - they're either US so shouldn't work on the UK version anyway, or just give the same problem - access violation and memory could not be read. I have not found a solution that works for me, so gave up trying to get RCT working on the XP machines i need it to work on.
I don't know why, if this IS an XP compatibility issue, it would be present in two such recent games. I have neither, so don't know if they work on my machine. Also, haven't tried RCT since installing SP2, which it's possible fixes the problem - for all the complaints, SP2 does fix quite alot of problems.
If you haven't got SP2 installed, give that a shot. The MS download site may still be bad for getting it, but AFAIK you can actually request it on CD from MS and they'll send it. Or you could not long ago (i got it on a PC mag cover cd =+)