if you're as leery as me about jumping on the vista bandwagon (at least for the next year or so before the first major service pack), now would be a GREAT TIME to try out Mac OS X.
And with
Apple Boot Camp, you can still have Windows (even Vista) running as a fall back.
On the Windows front, Dells are solid (I really like their phone support compared to others). I'm also a big fan of IBM (now Lenovo) ThinkPads. Been running the ThinkPads are work and we have some very idiotic users and they hold up great. And don't forget to defrag, defrag, defrag (99% of Windows users who say their PC runs too slow have NEVER done something as simple as defragging the HD. It's not a cure-all, but a little dab will do ya).