Well, that's easy enough. START > ALL PROGRAMS > ACCESSORIES > (COMMUNICATIONS > ) Remote Desktop (some XP installs have it just in the accessories, others in Communications). Run it, and put either the name or the IP address of the desired PC to remote in to, and connect. Now it's like you're sitting at said PC.
NOTE: You need to allow remote desktop connections on the remote PC. Right Click on My Computer, and go to Properties. Go to the Remote Tab, and allow remote desktop sessions (I'm currently on Windows 7, and don't have an XP machine booted and near by to tell you the exact syntax). Also make sure to go into the Select Users area and add any accounts that you may want to log in with. Take note that any account you wish to use for RDP (Rem. Desktop) MUST have a password on it, else the system will not allow it.
Kevin