Why not try this:
Take your boss' offer of the laptop and install both Truecrypt and VMWare / Virtual PC etc...
Make a truecypt container that is as large as your own personal windows installation needs to be, mount the container as a second drive; then make a VMWare / Virtual PC machine and save it to that drive.
Therefore you can keep the laptop clean, and still have your own environment to work in.
There is a time issue here... i;e; it takes longer to mount and load up the VirtualPC image than it would a normal windows installation... but it gives you good privacy.
Take the offer of the laptop, remove the hard disk... replace it with your own fully system encrypted disk, then when your boss yells "Timkins! Get in here! I WANT THAT LAPTOP BACK!"
You simply swap the original disk back in, and there is no problems. Your boss can't claim any ownership of the hard disk YOU bought can he?!