Well, this is precisely what Color Management is all about.
You can start learning about it here: http://www.gballard.net/psd/cmstheory.html
You need to calibrate and profile your monitor(s) accurately, regularly and often with a hardware calibrator puck.
Be prepared to read as much as you can on the subject. There are no shortcuts.
Wo Tai Lao Le
I think you may have scared BiiONiiC away
Learning about color management is a good idea if you have the time, but it sounds more than likely that this is just a difference in gamma between the Windows 7 machine at home and the mac machine at school.
If it's a school computer, the machine might be an older generation mac, and probably still uses a default display gamma of 1.8 instead of 2.2. Try this on the MAC machine:
Go to System Preferences > Displays > Color, then hit the [Calibrate...] button.
Click [Continue], then select 2.2 Television Gamma and click [Continue], then select Native or D65 and click [Continue], then name the profile (or leave with default name) and click [Continue], then click [Done].
See if this makes the images look better.