Get both sides' IP addresses (DOS -> "ipconfig").
Try accessing the other computer's files in your explorer by typing its IP address (\\IP_address such as \\192.168.1.2).
On WinXP make sure file sharing is enabled under the connection's properties:
- right-click your wifi's icon -> open network connections
- right-click your wifi connection -> properties
- make sure file sharing is enabled
That same approach applies in win7 as well, although the network connections are bit tricky to list (DOS -> "C:\Windows\System32\control.exe netconnections" for direct access).
If sharing is enabled, you might get a prompt to provide your login/pass (on the remote computer).
hth.