XP/Vista network problem

I have been having the same problem as many others: I can see the shared
files on the laptop from the laptop, but cannot access the desktop shared
files. I can access the internet from both computers.
I have Windows XP on the (HP) desktop, Vista Home Premium on the (Gateway)
Laptop, connected by a Netgear router. The only peculiarity with the setup is
that I don’t have an NIC card, so the router is connected to the desktop with
a USB/Ethernet adaptor.

I have followed every suggestion on fixing this that I can find, and have
spent hours on the problem with Netgear, as I assumed it must be the router.
The Netgear technician said that it must be something to do with the firewall
on the laptop.

I have:
Disabled Windows Firewall on both computers. I have no other firewalls.
Disabled Virus Checkers on both (Symantec on laptop, McAfee on desktop.)
Enabled file- and printer-sharing on desktop.
Enabled file-sharing on laptop.
Enabled NetBios over TCP/IP on both.
Installed LLTD on XP desktop.
Made sure the desktop ports for the network are open.

I have spent 2 weeks on this problem, and cannot solve it. Any suggestions
would be welcome!