So I am still at this, and I will never get the hours I spent on this problem back!!!
I decided to ditch the wireless router for now. Instead, I bought an 8 port switch and connected it directly to my wired router. I have now connected all PCs and NAS in the 8 port switch, and can successfully ping each from each.
But now, my HTPC, the trouble maker, still cannot connect to the NAS!!! It can now find it using the Easy Search Utility, but when I try to map the drive, it asks me for a Login id and password! WTF? None of the other PCs asked me for this! And I don't have any Login ids or passwords on any of my PCs or the NAS. I left the NAS as default -> admin, with no password.
Furthermore, I can access the NAS through the UPnP server and browser.
I'm really at my wits end with this thing! Any thoughts?