I agree, everything checks out to suggest you have the non-E P4C800 Deluxe. One more check that was so obvious I forgot about it is, that model is the only one in the series with a straight ICH5 chip. So I'll wager your big Intel Southbridge chip (largest chip in lower right corner of mobo) says 82801EB on the top instead of 82801ER.
That settled, use the
Asus P4P800/P4C800 Series: How to Update BIOS thread to reflash your BIOS. It should get your onboard LAN chip working, after which you can boot Windows and install the 3Com LAN driver.
Part of the instructions include loading defaults and putting your BIOS boot order back again; that's important, but is buried in the generic Step 12, where I say to "reapply your custom settings". If you use the correct AFUDOS tool, there should be no problem. The instructions also include saving your current BIOS, so you can go back again if necessary.
Hope this helps,
-clintfan