I wouldn't be too worried about it. Big box stores as the admin said don't have time to muck around in someone elses pc. And why would they have to, they could be charged for privacy.

Then again the store could have called you to ask for the user password to tell you they needed to check to make sure the power supply was working, or they could have easily tested it in another machine to confirm a working p/s and then waited for you to come in and check it. hmm... yes when you start thinking about it too much I see why you could be upset.

If you're really curious, instead of dwelling on the possibilities, Why don't you just phone them up and ask them why they left a knoppix cd in your drive ? See what sort of answer they give. You could even ask them why didn't they call you in to ask for a password? ...

Did you happen to sign any papers when you gave your computer to them? ... Should read the fine print if you did to see what you agreed (allowed) them to do with your system.

... in any case, maybe just pull the H.D when you send it in for something like that .. lesson learned.