Are you comparing images on the web between mac & pc, or the same image locally on each computer hard drive?
If your viewing via the web, there are many reasons why the look different.
The most important will be color profile. IE & Windows computers assume sRGB and ignore the imbedded image info. With Mac, and most Mac browsers, it will display the color profile of the image. If the profile is missing, it will revert to the default display profile & not sRGB.
This is important when viewing images from online hosting companies such as smugmug. They strip the profile out of the image when it is resized to save on bandwidth. I've had long discussions about this with the smugmug owners, and have been pulling my hair out. An out of the box mac install & safari will display washed out images when there is no embedded profile. To resolve this, I have designed my own site, and I'm moving all my galleries there.
I have done way too much research on this, so if you have any questions, let me know.
Dave