What he said.
Also it is very likely your router is using DHCP to assign IP's to your devices, but in case its not, make sure your addresses are all in the same subnet.
ETA: I prefer direct IP connects with shares, for example \\192.168.1.xx\c$
Not sure how the file system is on an Xbox, but alot of XP and Vista systems require extra options to be enabled for network sharing to work, like TekWarren said.