One PC cannot connect SQL Server 2005 Express but other do

I've an extrange issue with that. I'm working on a local network with 3 PCS running windows 7 pro x64. The fact is they need to be interconnected to use a program that is running over SQL Server Express 2005.

Ok, so i've the program in a shared folder and SQL express 2005 installed in one of them, then its supposed that the others only have to keep installed sql native client, and then open shared folder, double click on .exe and work.

it's running well in one of both "clients" and in a "server" (and we can work on both) but the third, is no cappable to connect sql database.

i tried to uninstall and install sql services, unable firewall and defender, enabling port on firewall etc...

When i try to connect to database from PC3 to PC1/SQLEXPRESS i get an error 26 (i tried almost everything i found on msdn) But when i connect from PC2 to PC1/SQLEXPRESS everything is ok. (same network, supposedly same installed referred to sql)

To make it more extrange, other day it was running, then we add PC2 to network and only PC1 and PC2 are working.

Any suggestions?

Thanks