Dear mcherry,
I'm no expert, but I think the trouble comes from the Apple gamma settings differing from PC gamma settings. Gamma refers to an exponent used to determine how bright to make pixels on a computer screen. The problem arises when different computer systems use different gamma values. I think the Apple gamma exponent is around 1.8 while PC gamma is way up around 2.4.
At any rate, I did a Google search on "mac and pc gamma" The first result was
http://www.cgsd.com/papers/gamma.web.html It kinda explains how to work around displaying your images for different gamma systems.
If you calibrated your Cinema display, the Monaco Eye-One system probably set the gamma to be at 2.2, or the "web average" as explained in the link. As for the uncalibrated monitor and graphic card on the PC, there's no telling what the gamma may be. If it's way up there, it can be as high as 2.5, which would most certainly make your images appear too bright.
What about other images you view on the PC? Do they tend to be generally brighter/washed out than the same image on the Mac (i.e. from other websites?)
All the best,
Dan