I've played around with a number of utilities for windows that tell you more about what's inside your machine and this is the one I like best.
System Information for Windows. By Gabriel Topala
For those who use macintosh computers it will look exactly like what you would expect from something with the name System Information... just like the MacOS control panel of the same name.
The best part is it doesn't need to install, so you can put it on a USB key, CD-ROM, or floppy disk (does anyone still use those?) and carry it with you from one PC to another.
An example of the kind of useful info you can get with it is just click on Memory on the left and it tells you how many slots you have in total, and what's in each one (memory type, speed, even brand!)