so both pcs are connected to the router.
ok try accessing your router settings 192.168.1.1 or 192.168.0.1 type those in a web browser no http etc
if it askes for username and password try admin, admin or if you know what it is then log in.
Once in just look at the basic settings is DHCP on? mac address filtering? whats its ip range set at something like 192.168.1.2-192.168.1.5 = 4 pcs connected at once.
Also on the pc that isnt working try start>all programs>accessories then command prompt.
once open type : ipconfig/release press enter then type ipconfig/renew press enter.
then see if it connects if it still fails in command prompt type ipconfig/all and paste the info here.
to paste in command prompt right click and choose mark then you can use your mouse to select text.