Check the System Properties menu. Right click the My Computer icon on the desk top and clip "Properties". This screen will tell you your CPU type, speed, and how much RAM you have. If running Windows XP, in the same box, click the Hardware tab and then click device manager. It will list all of the devices connected to your computer.

Another way is by running SiSoftware SANDRA. You can get this handy analysis/benchmarking program form Guru3d.com. Running the "System Summary" information moduel gives a quick snapshot of your system. Here's mine for example.