I just upgraded my Inspiron 1521 from Vista to Windows 7. I use Xming to log into my Ubuntu system from the Windows 7. It turns out that the Windows 7 firewall partially blocks Xming/XDMCP on my trusted network, even though it is set to permit all XDMCP and Xming traffic. When I turn off the firewall everything is fine.
By the way, I had the same problem with Vista. I solved it by trying a bunch of firewall products (including the Windows firewall) until I found one that worked. That happened to be the Sunbelt Personal Firewall that ins't supported under Windows 7.