Network Discovery / custom?

I'm running Vista Ultimate, and I'm having problems sharing
some files between others PCs in my network.

When in Network Sharing Center, I can't turn Network Discovery
to ON, just stay in "Custom".

I have turned Windows Firewall off, but still I cannot share some
of my files on my LAN.

I'll appreciate any help regarding how to solve this problem.

Thanks in advance,
Julian.