There are a number of things to consider here
1. For Remote Desktop, you MUST be running XP Pro - XP Home does not allow RD.
2. To log into Remote Desktop, the user account must be configured for access. Members of the Administrators group are automatically permitted. To add additional access, go to the Remote tab of MY COMPUTER properties and click "Select Remote Users".
3. Remote Desktop must be allowed in Windows Firewall if using XPSP2. This will open up incoming connections to port 3389.
4. If accessing through a router, port 3389 must be forwarded to local address.
5. If you are using a NAT router, most will only allow you to forward 3389 to a single address. If you have more than one machine that you want to remotely access, you need to change the port that RD uses. In registry, goto HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\TerminalServer\WinStations\RDP-Tcp\PortNumber and change the value (ie from 3389 to 3390). If you are doing this you will also have to allow that port in Windows Firewall. In the NAT router forward 3390 to the appropriate computer. Repeat using unique port numbers for each additional computer. When connecting to computer, using x.x.x.x:3390 to connect (otherwise 3389 will be used by default)
6. Windows XP remote desktop does not allow for interactive Remote Desktop. For this, users will have to send a Remote Assistance request (whole new can of worms), or, as noted above, a program like VNC is suggested.
7. VNC uses by default port 5900. Windows Firewall and routers will need to be configured in the same fashion to allow for 5900 to be forwarded to the computer.
8. If more than one VNC machine, change port 5900 to 5901, 5902, etc...VNC also refers to these as Screens. Access them using x.x.x.x:5901 - Again windows firewall needs to be told to specifically allow port 5901, 5902, etc.