Er your laptop is running wireless, but you also have it connected wired
One or other please.
Your wireless network will need to be in the correct mode - something to do with ad-hoc versus infrastucture. Get the wrong one and the router can only pass stuff to the net, not between computers in the local network.
Your PC that you want to allow the remote desktop in windows is going to need to permit such connections via its firewall. Under My Computer Properties > remote make sure the allow remote connections is active.
You should be able to just use the inbuilt windows remote connection: Start > Accessories > Remote Desktip Connection (at least it is that in XP Pro)