Wireless in a large house

I am trying to setup wireless in a very large house. I am useing Netgear 4 port router connected to cable modem. The access point is wired to the router. There are two PC’s on the second floor that are only about 60’and 70′ away from the access point. The 60′ PC has three more walls than the 50′ does. The 50′ PC gets a very good signal and the 60′ only intermitantly gets any signal. What can I do to “boost” the signal. I am very desperate, please help.
Thanks,
David