You can buy a clip-on ampmeter at Radio Shack. I'm guessing they're about 20-30 dollars. Then you buy a cheap extension cord, split the cord lengthwise for a few inches, clip on the ampmeter, and it gives you the amps of any appliance powered by the extension cord. Multiply that by your voltage, and you've got your watts.
If you want, you can go component by component. Plug in only the tower, and watch how current usage changes depending on your usage. For example, when you play a DVD, it uses more current. You can plug it into your monitor and see how much power it draws in standby-mode. You can plug in only your printer and see how much current is drawn as you print. Or you could plug in your whole power strip and read your whole setup.
And then you can use it for other appliances too, like the microwave and the refridgerator.