When compressing with some compressors (especially Sorenson) your video will get darker. It is recommended that you use QT Pro to compress with Sorenson and use the brightness + contrast filter to boost the brightness and contrast by 10-15% when exporting (Movie -> Video Controls-> Brightness). There may still be a difference in your monitor gamma and the PC's, but QT is supposed to compensate for that; somewhat.
PC's can usually play VCD's. I've had better luck playing VCD's and DVD's on PC than on my Mac. For one thing, video card makers often ship PC programs that exploit the hardware MPEG decoders on their cards. These little programs will play an MPEG encoded VCD or an MPEG file flawlessly, even on a slow computer. Before the G4, Apple used the hardware decoder on the video card to play DVD's on Macs, but, since the G4, Apple decided they had enough CPU power to avoid being tied to a hardware solution. (sorry, too much information)
Finally, software exists to create MPEG format files on a Mac which would also play on a PC (Toast Titanium comes with a simple one for making VCD's, which are MPEG-1 encoded), but you cannot make MPEG files with just iMovie/QT Pro. You MPEG movies may also carry the "darkness" problem. To correct that, I suggest you lighten your movies and tell your family to turn up the brightness on their monitor!
BTW: Have you calibrated your monitor?
