Hi there
You will have to setup a ADHOC network between the 2.
On the Vista machine (HOST):
Click on Start -> Connect To.
Click Set up a connection or network.
Click Set up an ad hoc (computer-to-computer) network.
Ont the XP machine (Client):
Click Start -> Sontrol Panel -> Network Connections
Right Click on the Wireless device and select properties.
The wireless Network Connection Properties will appear.
Click Wireless Networks tab, here I tick Use Windows to configure my wireless network settings.
After that click Advanced button.
Note: You can also use the configuration tool provided by wireless adapter manufacturers to configure ad hoc wireless network.
Wireless Network Properties
Advanced window will appear. Select Computer-to-computer (ad hoc) networks only option. Click Close at last.
Good Luck
Please let me know of this helped.
Cheers
Grant