You must be sure that your app is not causing the problem and it seems you can be sure in Windows XP the error is not being generated. So this moves the problem to Windows 7, so start from there, do some tests using windows 7(using a 100% trustable pc or laptop configured by yourself) in a computer near to the server (local cable not so long) to define exactly when the problem is being generated. If none, then continue increasing distance (using other routers), this way you can find if the network has issues.
If no problem is detected this will moves the problem to the local machines. Maybe is a software installed on the network that affects your app.
If you find problems you can decide what to do.