Windows Vista for me is a must-have upgrade for many different reasons. But some of the reasons it's so compelling are not being trumpeted enough.  Windows Vista includes with it a host of new tools for knowing a lot more about your computer than previously available.

For starters, open control panel:

Under programs look at "Change start up programs".  Under XP there was MS Config but Windows Defender is much nicer about this. Now you can actually control what is getting loaded up on your computer. 

As you can see here, I can now see what loads on start-up. The most common problem normal users run into is that after a period of time, their start up gets clogged full of junk and managing this can be really tedious.  In Vista, that problem goes away thanks to the integration of Windows Defender into Control Panel.  More advanced users will probably want something like TweakVista but as something that you know is there out of the box, this is very handy.