Get Aida32. It is the ultimate system info utility I have come across.

You can create a COMPLETE report and find out EVERYTHING about your PC.

You won't be disapointed... its FREE.

Download it here.

Edit:
I find that the enterprise version (also free) gives a bit more info. You can retrieve license info for your m$ products along with cdkeys and etc. I like it much better. Its also nice for on the go. If you do side work and you have clients all over, you can put this program on a usb flash disk, then create reports and store them on the flash drive.
When your client calls... no need to ask them what they have... simply plug in and find out for yourself.