On the Mac, there is a hugely useful iStat Menus. It displays all kinds of statuses, such as CPU/RAM/Disk utilization, HD temperature, etc...

Is there something similar for Windows XP?
Edit: I can't believe how crappy and unaesthetic most of these are compared to iStat Menus. Most of the tools mentioned seem to focus on secondary features like skinning and theming rather than being unobtrusive, functional and out of the way. Moo0 SystemMonitor probably comes closest to being functional and easy to use, but it is totally obtrusive and in your face.
