goto Start/all programs/accessories/ command prompt
type ipconfig (you will need to do this on both pcs), take a note of the ip address (usually 192.168.0.**) ** can be anything between 1 and 255,
then if the laptop is 192.168.0.100,
type int command prompt on the Desktop pc "ping 192.168.0.100" +press enter
it should take a minute, hopefully it will report success (4 packets sent, 4 packets recieved 100%)
if so then the network is ok.
if not then you might need to run the networking wizard again.
have you enabled/installed file and printer sharing?
lastly, have you got the firewalls enabled on both pcs, if so try turning them off one at a time starting with the laptop.