Tip: Path Variable Expansion in Vista
I was just installing some beta bits and it required removing some files from my temp folder %temp%.
I didn't realize until this weekend that you could type a path variable, like %temp% into the Vista address bar and it will just work.
I never tried that in XP, but it probably works there as well.
Prior to discovering this, I used to hop out to a command prompt when I wanted to know what a variable resolved to. I used to do a lot of DOS programming way back, so when I think of path variables, I immediately think command prompt.

I know you can go into Control Panel, System Properties, Advanced, Environment variables, but that's a lot of clicking for something simple.
I've read that most folks only use something like 10% of the features in Microsoft Word. I suspect the operating system functions and shell niceties have a similar usage rate.