Don't take your pc for a healthcheck I work for PC Service Call, we do all the desktop support for PC World, Dixons and Currys customers.
I'm not sure what they do (they should run a disk which basically has antivirus, anti-spyware and a chkdsk & memtest utility)

They normally end up blocking user's access to the internet (through norton) which we have to undo. They will also more than likely slow the pc down, and mean that you have to do a product recovery which make's your £50 worthless.

Mind you, some stores complete them successfully (not found one yet though)