To be honest you can't unless you contact them and see if they imaged your drive before they trashed it.
The service guys will have squirted an image onto you drive and removed everything on it.
The reason why they do it is becuase its a zero input method of testing the computer they just hook it up to a network and leave it runing for a couple of hours and the scripted software will say pass or fail after its done its thing.
They will also come out with some persudo legalise rubbish about how they can't return computers without confirming that all software on it is legal.
Personally if I was you I would use this pain in the neck feature to go Windows 7.
Next time you let anyone near your PC always take a disk image. By that I mean a complete image of the drive not just a backup.