Make sure ya don't have one of the PC's hooked up to the "uplink" port on the router, most routers only have a 10mbps uplink port as that is all that is really needed for the cable/adsl modem. So yeh, if u've got a PC hooked up to that then could be the problem maybe?
Also check using the latest drivers for your nic's on both PC's.
Also check the cable is in good condition ( & make sure its CAT5 or better)
JayMan