Remote Assistance

having trouble getting the remote assistance to work between 2 users… they both are using windows xp pro and they are both still on sp1… when you click on the email link and enter the pass it says connecting then comes back and says the remote assistance could not establish a connection because the host name could not be resolved…have tried it through messenger as well and get the same error.. have reversed it and tried to remote the other pc and get the same error that way to…any suggestions on why this is not working and connecting? thanks