CO2SAVER, a free program for Windows XP and Vista seems to have solved the problem.

http://co2saver.snap.com/

CO2SAVER, a free program for Windows XP and Vista seems to have solved the problem. It gives you a simple pre-configured control panel that specifies when to put the machine (and attached monitors) to sleep. It also tweaks some hard-to-configure settings that, according to its developer, Lee Hasiuk, defeats Windows attempts to keep a machine awake if it thinks (correctly or otherwise) that it is detecting a background task other than mouse or keyboard activity.

It can save over 2,500 lbs. of CO2...

If you're buying a new PC, check out a laptop as they often use just 20% of the power of a desktop PC & monitor (even with a big 17" screen).
Apple's all-in-one iMac is also power-thrifty even the giant 24" screen version. (Macintoshes don't need CO2Saver as they already go to sleep when inactive.)

And don't forget to turn your TVs, VCRs, DVDs and stereos OFF. They could be costing you over $250 per year per year per set.