Sorcha - I know a number of people plagued with this symptom. One one model of compaq it turned out that we needed to "flash" the bios with an updated version. (your tech should know what I mean) - for some of the others we just have to routinely and frequently clean the registry of invalid entries - (we use either reg-clean or easy-cleaner)

It doesn't sound like a corrupt registry to me unless when your "doc" tried a new drive he imaged from the old one rather then a clean install. But thought I'd mention it anyway.