Windows XP and Vista both have the capability to 'Hibernate' when you don't
need your PC and this is really, really good news for you and the environment
because while in hibernation your PC uses very, very little power.
The other very useful feature for many users is that you don't need to
close down all the programs that you were using before you hibernate.
Even more useful is, as you might expect - you don't have to wait for your
PC to go through the ritual of starting up again!
If you want to save the last few Watts of power then
you can switch it off completely at the socket on the wall while it is in
hibernation mode - Laptop users don't
even need to do that last action - just unplug the charger.
Please note that you fully SHOULD CLOSE DOWN your PC at the VERY
LEAST EVERY MONTH** and my normal recommendation is every week(end).
Note that every Windows PC needs to be restarted after the
ritual of Microsoft's 'Patch Tuesday' (2nd of every month) for important
security updates to take effect. See Windows Update - Settings and Advice for recommendations.