Possible Reason are:
I.They didn't install TCP/IP.
II.Pls Check wheather they've any anti -virus software (eg. PC-Cilin) , if yes, pls ask them to
disable the Fire-Wall or ask them to un-install it.
III.If they are using Win XP, try to turn off the internet Connection Firewall
1.Right Click 'My Network Place'
2.Right Click 'Local Area Connection'
4.Untick 'Internet Connection Firewall'.