Internet via DSL but no LAN

Help: Internet via DSL but no LAN

I have a new PC, from a TigerDirect kit, using
the BioStar M7NCD-Ultra

It is nice. But one (1) weird problem remains
after a week of struggle.

It will access the internet via its on-board Ethernet
adapter thru my DSL, BUT when I enter "Network PLaces"
and then try and enter "Entire Network" I get this error:
"Windows is unable to gain access to the network."

This also occurs when I install a PCI card Ethernet adapter.
Internet access for Web and email but no LAN to my other PCs.
Same deal from a fresh install of both Win-XP Pro AND
Win-ME. Same when the adapter has no cable, cable to live
or un-powered D-Link router, any of the 4 LAN jacks;
and with and without the DSL plugged into its WAN position.

I've got newest drivers for the Mainboard and no issues in
Control Panel \ Devices. I've checked and played with the BIOS
over and over and reset defaults...

Sometimes, If this PC is plugged into the router it interferes
with the other PCs on my home LAN such that they may not
see each other or they can't enter "Entire Network" within
"My Network Places." But they too will continue to get
greater internet access via the DSL.

Is this Crazy or what ? My thanks and the words
"You're a Genius !" await a solution