Timed out error when windows XP tries to connect to sqlserver r2 2008

My windows form application is developed on vb.NET using SQLSERVER R2 2008 Express as database.I installed sqlserver r2 on windows server 2008 of Client Location.There are about 8 systems on their network.Some of PCs in the network have Window7 OS and remaining have windows XP OS.My application is connecting to database using connectionstring name :Servername/instancename.I have switched off the firewall of server where database installed and on all the PCs Of network.Pcs have windows7 OS allowing connection to database.But,The Pcs with XP is displaying timeout error.using UDL test,I can access the instance of server,but when i try to choose the database,they are getting same time out error.Please help