After a couple of month with Windows Vista i found that the default behavior of the start menu Power button anoyed me alot, especialy on my Desktop pc. I was researching and playing around and found how to change what the start buttons do.. You can change how the shutdown/sleep buttons function in local group policies (gpedit.msc) COMPUTER CONFIGURATION > ADMINISTRATIVE TEMPLATES > SYSTEM > POWER MANAGEMENT > BUTTON SETTINGS
Changing these values will resolve this idiotic default idea that MS came up with of pressing the power button and it going to sleep....
Another way is in the advanced power settings.
I hope that this can help you out a bit.