Network connection issue - PC to PC (unusual)

I have three pc's on my network, two boxes wired and a wireless laptop; networked by a netgear fwag114 wireless vpn router. All are running Win XP Pro SP2. Has been running normally for 3 weeks since I got the new router - file sharing was working, all network connectivity was fine. Until two days ago...

Suddenly my laptop stopped connecting to my main box, but not my backup box. My main box connects to the backup box and vice-versa. My laptop connects to the backup box and v-v. My laptop and main box can see each other on the network but will not connect to each other ("\xxxxx is not accessible, ...blah blah... Network path not found" ) All machines have internet connectivity still... here's everything I tried so far:

--- Machines were rebooted
--- Double checked workgrouping and machine naming
--- Double checked IP subnet settings (All were under 192.168.0.x)
--- I have shutdown all firewalling on the pc's in question and got the same results
--- I have tried disabling the wireless adapter on the laptop and using the wired adapter and got the same results
--- File/Printer sharing for MS Net's is there and checked on each pc
--- I've tried connecting to specific shares thru the Run... dialog and no bones
--- I have tried re-running the network setup wizards on each pc, to no avail.
--- I have used system restore on the main box and the laptop to b4 this started, no change.
--- Now this is where I got lost--> ping between the main box and laptop does not reply in either direction; however a ping from the backup box to the others gets a reply, a ping from the router to the networked pc's gets a reply AND both the main box and laptop can ping the backup box with replies... 8O :? :x

The results from my ping test really have me confuzzled :? . I've never seen something like that before! I was sure it was just a problem somewhere with a driver setting or something, but that threw me. I don't even know where to go from here and Microsoft support was of absolutely no help :x . Anyone have any ideas, or seen something like this before?

Thanks in advance for reading this and for posting if you do...

Yes, NIS 2006 is running on my laptop. Even though I've disabled it, does its presence cause that effect? Hmm, I will try updating it and if that doesn't work then uninstalling it and see what happens... thanks!

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OK... I tried updating NIS and still the same problem exists. So then I uninstalled NIS and returned windows firewall to its original pre-install state and still no change in the problem. One more thing to add to the list. I'm almost wondering if the issue is on the main box... if somehow permissions or an account got its settings changed, but I don't know much about permissions and user accounts though. All I know for sure is that I didn't change anything myself, and until 3 or 4 days ago it was working fine. Please help :?