If a wireless interface isn't up and running with an access point (or another ad hoc system) it's considered "unplugged".
Is it possible your wireless access point is on the WAN side of your desktop router, and the windows sharing is being filtered/firewalled?
Where's the WAP?
Code:
Desktop-\
>router?<
Desktop-/ \-WAN port with Cable/DSL Modem?
Basically, what's the topology? What's connected to what? Which are routers or access points starting from your DSL/cable box? Multiple private network ranges?
I don't want to jump into the middle of things but the posts above leave me with questions. It's always easier if we can "see" the net.