Strange remote desktop connection problem.

I have two PCs (Win2003 and WinXP) on the LAN. I can connect from one to the another using remote desktop. Here’s the problem. I can connect to the Win2003 PC from outside the LAN but cannot connect to the WinXp. Both use public IP addresses. When I swap their IP adresses I still can connect to the Win2003 only, and cannot connect to WinXP.
It worked before and stopped suddently without any reason.