How to set up a compile farm using your unwitting windoze colleagues' machines

The motivation:

I work in an environment that's full of Windows folks. Most of them use their computers for little or no CPU intensive tasks. So it's mainly check mail, use Word etc. I, on the other hand have a laptop that runs gentoo and who desperately needs CPU cycles to take me beyond the computing power offered by my 2.2GHz celeron processor. :(

So after a lot of thought.