Windows XP to Windows 7 Networking

Customer had eight machines, one is a PC server serving one application that uses SQL. Recently they purchased three new machines replacing two work stations and the PC server. Everything works fine between the Win7 machines and the PC server. Originally I was unable to connect at all between the XP machines and the PC server. The tech’s at the software company got them connected by logging on to the PC server and one work station remotely. That lasted one day. I went back in played around and hey started working. That lasted about one day. Now none of the XP machines will connect to the server. They can see the server in the network list but can not connect.

What am I missing?