Can anybody shed light on this problem?

 

I am unable to set up a functioning home wired network.  XPS 420 Internet connection through my router is extremely sluggish at best, usually non-existent.  Connection to other computers on my network is only possible if I disable all firewalls.  My normal setup is Norton Internet Security with firewall enabled, Vista firewall disabled and router firewall disabled.  I can connect directly to the Internet through the modem (bypassing the router) with no problems using Norton firewall.  The router passes the Microsoft Internet Connectivity Evaluation Tool tests (I keep UPnP off in the router however).  Going to a "clean" boot (no non-Microsoft services and all startup items disabled) doesn't help.  It doesn't improve in Safe mode either.  I installed LLTP (Link-Layer Topology Discovery Responder protocol) on my XP machines and they do show up in the Vista Network and Sharing Center (View full map) but not in the Network window.  The XP machines all connect to each other and to the Internet with no problems.  All machines are in the same workgroup.  The fact that I can network succesfully with all firewalls down would seem to indicate that permissions and sharing are set up correctly.

XPS 420, Intel Core2 processor Q6600 (2.40Ghz,1066FSB) w/QuadCore Technology and 8MB cache 
3GB DDR2 SDRAM at 800MHz 
Dell 19 in 1 Media Card Reader, Keyboard, Mouse included in Dell Bluetooth Package
512MB NVIDIA GeForce 8800 GT 
Serial ATA Dell DataSafe With Dual 320GB Hard Drives 
Microsoft Windows Vista Home Premium Edition, English 
Norton Internet Security 2008
56K PCI Data Fax Modem 
48X Combo and 16X DVD+/-RW 
Sound Blaster X-Fi Xtreme Music 

SMC 7004VBR router (wired)