Willy Tee,
I just saw this post and I feel ya! I just spent an entire day trying to figure out the same exact problem.
I have a home network with a wired XP Media Center desktop for the main computer and two wireless Vista laptops.
They all played together beautifully with my previous router, but couldn't see each other with the D-Link. I proved this by swapping the old router and the D-Link back and forth and changing nothing else. Old router worked everytime, D-Link did not.
Wireless Vista laptop 1 could "see" Wireless Vista laptop 2 but 2 could not see 1. And neither saw the wired XP Desktop. The XP was in a world all its own. I tried to look at Workgroups on the XP and got the same message you did, "not allowed".
I checked a lot of the same things you did, including the workgroup name -- several times. Nothing seemed to work.
Finally I did 2 things back to back and everyone saw everyone else on the network.....
1) I reconfigured the Router settings and THIS time under DHCP Server settings, I turned on "Netbios Announcements" ("NetBIOS allows LAN hosts to discover all other computers within the network, e.g. within Network Neighbourhood.") and enabled "Learn Netbios Information from WAN" ("If NetBIOS advertisement is switched on, switching this setting on causes WINS information to be learned from the WAN side, if available").
2) I went to the firewall settings on each PC, pulled up the network and declared each member of the network "a trusted source" - again I repeated this on each PC.
Now I don't know which of these actions made things work and I don't care. I just know I now have a fully connected network!
I know its been some time since you posted. I hope you either already solved the problem or if not, that this will help you.
T