I recently had this same problem, but I found the solution. It was pretty weird. If my firewall was on, I couldn't receive any packets, but if it was off, the NIC worked just fine. (You're probably thinking "this guy's an idiot and had his firewall configured wrong" but that's not the case, here me out. If I popped in a different NIC, and disabled the onboard, the firewall would work just fine. )
The problem was with the default configuration of the onboard NIC driver which can confuse some firewalls (at least it confused Sygate Personal Firewall Pro). To fix this problem, go to the properties of the 3com onboard NIC in device manager (or if you go under "network connections" then click "configure" to get to this page), and on the advanced tab, disable any and/or all of the following:
Offload Checksums
Offload TCP Checksums
Offload TX Checksums
Offload RX Checksums
These are all basically the same thing, and the way it shows up on your device manager may depend on the driver rev. Once this is disabled, I bet your NIC will work just fine.
The purpose of this "offload" option is that normally, the CPU will calculate TCP send (TX) and receive (RX) checksums, but when this option is enabled, the network card does it instead, which is supposed theoretically improve performance by taking a little load off the CPU. The problem seems to be that for some reason, the checksums generated by the NIC are slightly different than those generated by the CPU (different algorithm?) and the firewall seems to expects to see them in the same form the CPU would calculate them, and when it doesn't, it thinks the packet is bad and drops it.
It was a really weird problem and I didn't find much info out there on the net about it, so I thought I'd share it a few places to let this fix be known. How did I find the answer??? By calling Sygate's sales department (where they tell you over and over while you're on hold: "this is not a tech support line") and I told them I was thinking about buying Sygate Personal Firewall Pro, on a friend's recommendatation, but that he upgraded his motherboard to the same motherboard I had and he was having problems. I figured they might be motivated to find the answer if it might involve a sale.
Anyway, now that I've got that fixed, my P4P800 Deluxe is working great! The other day, I cranked my 2.6GHz P4 up to 3.38GHz and was able to keep it there long enough to run a few benchmarks, before I had to shut it down and go eat dinner.
-Skier
