It's very simple.

On the 'Server" machine, the one that will be accessed via remote desktop, go to Control Panel -> System -> and in the system window select the Remote option (in XP it's a Tab), In the Remote Desktop section check the checkbox for "Allow users to connect ot this computer remotely." The two machines will need to be connected to a network, of course, and you will ned to ensure that the "Remote Desktop" option is checked in the Firewall settings (to allow the RDP port to be opened in the firewall). then in the other machine the "Client", the machine that will access the 'Server's" desktop open your menu -> All Programs -> Accessories - > Remote Destop Connection program (it may also under Communications). In the prompted area enter the name of the computer you want to connect to, and the username and password when prompted. You will then be able to access the machines desktop, and it's fielsystem.

here's a nice help page I found Get started using Remote Desktop with Windows XP Professional