I believe that Windows Vista is the equivilent of taking a huge dump on your computer's CPU. Windows XP was decent, but OS X is much better.
Here's just a little example of how inefficient Vista is:
OS X and Vista are pretty equally graphically intensive (colors, animations).
I have an iBook with 256 MB RAM and 900 MHz PowerPC G3 running OS X Panther.
My girlfriend has an Acer Aspire laptop with 1GB RAM and a 2 GHz Intel Core2Duo processor running Vista Home Premium.
I was at her house and we were working on the computers. We were both playing music while web browsing. Her Vista Acer locked up 2 times when trying to change tracks. One time it just went to a grey screen and we had to reboot manually. My computer ran like butter throughout.
Even with much less processing power and less RAM, my Mac did exactly what her computer did with no hiccups.