PC only networks in one direction

Hi guys. I have another problem with my office network. Four of our computers are on the same network. All can see eachother, except that the computer at our reception desk can't see one of the PCs, which I'll call Mary's PC. Now Mary can see the Reception PC, but Reception can't see her. That's the only blind spot in our network.

So I tried typing \\Mary into the Reception PC, and it says that the network path cannot be found. I also tried \\ipaddress, and that didn't work either.

Incidentally, if I type \\Mary on any of the other PCs, it comes right up. But if I type \\ipaddress on those PCs, nothing happens.

And finally, if I try pinging "Mary" on one of the computers that can access her, it says that host cannot be found. But if I try pinging her by IP address, it says "request timed out" 4 times, then gives 4-0-4 (100% loss).

So, something is screwy with Mary's computer. We can't ping her by name, but two of our computers can see her in the network places and access her. Reception can't see her at all, and all computers that try pinging Mary by ip address get a time out.

I did try unchecking the bottom 2 boxes on Mary's PC's TCP/IP DNS tab ("register this connection's addresses in DNS" and "use this connection's DNS suffix in DNS registration"), rebooting, and then checking them again. That didn't help. Also, her Windows firewall is off, and she has no other firewall running.

Any ideas?

thanks,

-Emery