Good work stevieg with a car like mine this will be very handy as the mechanics like to take it for a test drive - OBDII recording would come in handy too to keep an eye on their driving.
I presume that you only intend to have this on when you take the car in for a service but it could also be handy for if the car is stolen so what about using an RFID tag or one of those USB lock keys on your keychain to disable this software? But then that could be considered getting a little complicated and ruining a nice simple bit of software. You could have it triggered by the valet switch into a serial port too - ie have the valet switch not to disable the PC but put it in surveillance mode. (I apologise, that sounds like something from Knight Rider!)
Oh just thought of a few more things: could it be set to turn off the monitor after say 1 minute (and it wil then come back on when the screen is touched) so mechinics dont try and mess with it. And would it be best to have an on screen warning as recording people might be considered slightly illegal and if they see a warning that they are being recorded then they may not do the stupid things - prevention is better than cure.
Then again I suspect a dodgy mechanic I would take it badly and do a half assed job if he knew he/she was being recorded.