remote desktop with l2tp vpn dialer - HELP!

Hi all,
i am a very determined amateur failing as usual to let go of a bone i just can't gnaw thru.
i have been trying this past week to set up remote desktop for use between to pc's both running win xp pro and connected to the web thru cable connections. i finally managed to get it working one way (sadly, the less important way) after i realized the other pc was connected thru a router and tweaked that. still unable to connect to my pc from away.
i think the problem might involve the fact that my internet connection is as follows: i have a network card connected to the cable modem (motorolla sufrboard) and that connection is defined as a LAN. in order to actually establish the internet connection i have a second network connection that was installed by my ISP. it is defined as a VPN - Wan Miniport (L2TP). this is what i click on to connect. my isp insists i use this. could this be the reason i have not been able to connect with the remote dektop application? also, when i run ipconfig in a command prompt i end up with two ip adresses lines - one for each connection, which is the right one (i have of course tried both by now many times).
can someone help - i really need to sleep again soon!
eli