Umm maybe a daft question but how is your wireless network structured?
Are you connecting both PC's via a wireless router so both laptop and desktop use this to share the internet connection out?
or,
Are you connecting the laptop directly to the desktop and using ICS (Internet Connection Sharing) on the desktop?
Wi-fi networks can also operate in a couple of different modes: Infrastructure network mode or Ad-hoc mode. The former has a master node, usually a router and it controls the network dishes out IP's etc. The latter allows all PC's/devices within range of each other to talk directly to one another, so there is no master controller as such running the network.
Need a bit more info I'm afraid before I can comment/attempt to help.
*EDIT* However, if I had to guess, check your XP firewall settings to make sure that either your desktop or laptop isn't 'playing hard to get'. I'll also assume that you are trying to use your desktop to let your laptop access the internet, in which case you'll have to go to your wireless internet connection properties dialog box on your desktop and enable ICS for that connection. From memory something like...
Click 'My Network Places' maybe on your desktop or from Windows Explorer.
Click 'View Network Connections'.
Right Click 'Wireless Network' icon and select 'Properties'
In the properties window select the 'Advanced' tab and there you should see options for the Windows Firewall and also for 'Internet Connection Sharing'.
Hope this helps, assuming that I've assumed correctly
S.