I use TightVNC for some client connections. It gives me remote access and control of a PC. They have to set up the VNC server piece to allow me to connect (and give me an ID and password), but it does give me full control of the machine involved. And I mean full -- I can reboot the machine, change registry settings, download files, or anything else I want (and am allowed to do, permissions-wise).
I'll go back to my prior question: what kind of work do you do? The reason I ask is that you can turn it around, explaining that in order to do your job, you need to be able to do X, Y, and Z, and that you can't now for the following reasons. Then fill in the reasons.
Also, are you hooked into an office or another environment? you could get nasty and stick a little firewall on there that doesn't let anything out...
Do you have proof of anything that he's downloaded from your machine that he's not supposed to have? I'm thinking of things like emails, files, and so on. If you do, these are powerful FUD-creators for the corporate bigwigs you mentioned. (FUD, BTW, is Fear, Uncertainty, and Doubt). Explain that he's got copies of these files from your machine for no apparent reason... what files may he have from other machines in the company? Contracts? Proposals? Customer lists? Salary reports? Performance reviews? Company plans? The list can go on and on.... and the realization that they could be next can be quite... iluminating...
Feel free to PM me if you want to talk it over. There are ways to secure your system, protect yourself, and launch an offensive to get to where you want to be.
Oh, yeah, one last thing: is this for the new job or the old one? 'Cause it would really, really stink if this were the new one...
