You can use remote desktop. It's built into XP and works quite well.
On the machine you want to control right click on "my computer" and select properties. There will be a "Remote" tab. Check the "Allow users to connect remotely to this computer". If you log on with an account that does not have administrative access you will need to allow that user to login. If you're using the default XP security (there really isn't any) then you should be good to go.
On the client machine, the remote desktop client is under Accessories -> Communications -> Remote Desktop Client. Bring it up, enter the name of the machine you want to log into and you're good to go. Note that it will disable the keyboard on the remote machine while you are connected.
If this doesn't work there are lots of help pages on the net about using it.

mike