Windows 7 has a great tool built-in called Windows Easy Transfer, that can grab your data from an old computer even one running Windows XP and transfer it to your new machine with an external drive. To set it up, plug in your external drive and hit the Start menu on your new machine. In the search box, type "Windows Easy Transfer" and click on the shortcut when it pops up.
When it's done, unplug it and plug it into your old computer. If you don't get an autorun prompt, head to My Computer and double-click on your external hard drive. From there, it will ask you if you want to install Windows Easy Transfer on your Windows XP machine. After it does, it will scan your computer for data to transfer.
Select the users you want to copy over and hit Next. Save the data to your external drive or flash drive and wait for it to transfer ( this might take awhile ).
If your old computer is running Windows 7, you can skip all the above steps and just open Easy Transfer directly from the Start Menu's search box and picking " This Is My Old Computer ". It will then perform the above steps and transfer your data to the external drive.
Select your drive from the list and check the users you want to transfer over. Wait as it transfers all your data (again, this could take a few minutes). When it's done, your computer will restart, and you should see that all your files and settings are available on your new machine.
Note that this won't migrate any of your programs, just your settings and documents. It will, however, let you know what programs you had on your old computer so you can write down a quick list of the programs you'll want to reinstall.