Two Computers On Network Not Communicating

I have a very small network consisting of four wired, networked computers. Three of the computers are running Windows XP and one computer is running Windows 7. They are all set to the same workgroup name, file sharing and permission are turned on for all four computers. They are connected through a D-Link router. The three XP machines communicate flawlessly with each other, and two of the XP machines communicate flawlessly with the Win 7 machine. However, one of the XP machines cannot communicate with the Win 7 machine. I pinged the Win 7 machine from the XP machine that is having the communication issues, and all four of the packets went through. However, when I ping this XP machine from the Win 7 machine or any of the other XP machines, only one packet in four goes through. Could I have a bad network cable or NIC in this XP machine? If so, why would it continue to communicate with the other XP machines? You'd think with only 25% of the data being sent successfully that it would not be able to communicate with any of the other computers. Also, we have had this arrangement for over a year, and this is the first big issue we have had with the network.

Never mind. Got it to work. Manually set the ip address on the XP computer that was having issues with the win 7 computer and that put them on speaking terms again. Changed the NIC back to assign address by DHCP. It assigned it the same ip address, but the network is still working properly. Previously I had tried to repair the connection through XP's control panel but it did not work. You would think it would assign it to a new ip address instead of renewing the old one. Guess there was something in that address that could not be flushed out until it was inactive for awhile.