You may have heard claims about how Vista can help you save electricity. It has 3 features which play a role in this. Firstly it's "Sleep" and Hibernate modes work better than their equivalents in XP - various things have lost their veto over Hibernation.Secondly the machine can "wake" from it's sleep state to run a scheduled task, so you don't need to leave PCs on overnight for software updates, disk de-frags, virus scans and the like. And Thirdly we've added power to the set of things which can be centrally managed with group policy, so PCs which are left on 24/7 and spend most of their time running screen savers can now be powered down.
The numbers go like this, There are 8760 hours in a year, and a PC only needs to be on 10 hours per day 5 days a week = 2600 hours a year. So those 24/7 PCs could be in sleep mode for 6160 hours per year. [That's the basis we use, and I think the savings are bigger - a PC isn't used from 8AM till 6PM in most offices, it can power down when we're away in meetings or at lunch, and we don’t come into the office 260 days a year... you might have a further 1000 hours of sleep. Those PC produce heat - reduced it, and you reduce air conditioning requirement too]A PC and monitor use about 125 Watts when running and 5 Watts when on standby a saving of 0.12KW. Multiplied by 6160 hours per year that’s 740 KWh per year. The national energy foundation have a useful "calculate your CO2 emissions" page which says 1 KWh of electricity makes 0.43 Kg of C02, so 740 KWh is about 1/3 of a tonne of C02. Multiply by the number of PCs you have and it's quite a big saving.
I found a paper from the UK parliament which makes that about 3% of the average C02 emissions per person here - France emits less Co2 than us - because their electricity is mostly nuclear, so their Co2 saving will be less. US emits twice as much (4.6% of the worlds population contributes 23.8% of green house gasses) so they need all the savings they can find.
It's hard to estimate the number of "Sleep hours" we can get (how many PCs run 24/7 ? What can it be reduced to ?), and I don't have a world wide average for C02 emissions per KWh, and I can't predict how much power an average PC will consume in the future ... but with hundreds of millions of PCs, saving hundreds of Kilowatt hours, that makes tens of millions of tons of C02 - equivalent to whole output of a small country. Doing something about global warming means both big changes (like electricity generation) and small ones (in its consumption). This is a small change, but as the Chinese proverb has it, "The Journey of a thousand miles starts with a single step"