Have you checked the connectivity of the network?
i.e. used command prompt to ping
computer1 to router
computer2 to router
computer1 to computer2
computer2 to computer1
I recomend you do this as it doesn't look as if t is a roblem with the router.
[to do this, goto start, click run and type "cmd"]
[once in cmd, type "ping {ip of computer/router you are pinging}"]
The computer to router pings should be sucessfull (if not then

)
If the computer to computer pings then fail, try disabling any firewalls you have (check the windows firewall) and try the pings again.
Do this frst before worying about file sharing... conectivity is much more important
