Network Problem of my 6000 with Desktop

I have two destktop computers (Dimension 8400 w/XP MCE, Dimension 4550 w/XP Pro) and two laptops (Inspiron 8600 w/XP Pro and new Inspiron 6000 w/XP MCE). I need to setup a new network for my new Inspiron 6000. I followed "Network SetUp Wizard" and successfully add 6000 to my network. But, the problem happens: not only 6000 but also other computers cannot access the files in 8400 (this computer is connected directly to my DSL modem.. and other computers are connected wirelessly via Netgear router). Odd thing is that 8400 can still access the files in other computers. So, the problem is not both ways; other computers cannot access files in 8400.

 

I did not have this problem before I set it up. But 8400 network was unstable (sometimes I need to reboot the computer to enable to acess the files in 8400). I tried to re-setup a network at 8400 (I am wondering which option I need to choose...

when I setup the computer connection...I chose a "residential gateway one" and "directly connect to the internet and other computers are connected through this computer" but no go. In fact both options are not correct. Because 8400 is directly connected to DSL modem and other computers are connected via Router. DSL modem is directly connected to Router. I am not sure if this section cause the problem. Please advise me how to resolve this issue.

 

I contacted DELL software support (since I purchased laption within 30 days), but they said that they no longer offer 30days free software support.