There is a winsock fix built into XPSP2, so you don't have to mail the fix to them.

They have to have SP2, and to check that go to Start>Run>winver

On the line that says Version 5.1, at the end it will say what (if any) service pack is installed. If it says Service Pack 2, then you can continue. Otherwise, mail the XP Winsock fix to them.

Run a command in Start>Run>cmd

Type "netsh winsock reset catalog" (without quotes) and that is the equivalent of the winsock fix. Windows will have to be restarted, though before it will work.