Gigabit Network Compatibility

I'm building a new computer - newer than my current 5-year old dinosaur (which has been upgraded as far as it will go - the only free space inside is the ISA slot.)

I now have most of the bits assembled and installed a working system, but I cannot get it networked.

The old system has a bog-standard 10/100 NIC connected to a small but feature packed black(ish) box which is an ADSL modem, firewall, router, dhcp server, 4-port 10/100 switch.

The new system has a relatively new (but second hand) Asus K8V-X mainboard with onboard LAN, but I cannot get it to talk to the switch. The switch has the usual pair of "link/activity" leds per port. The pair connected to the new machine flash simultaneously and briefly every 2 or 3 seconds, but are otherwise off (even the "link" light.)

The old machine works normally (link light always lit, act flashing when there's data flowing.) I've tried swapping cables and ports and both together - all with the same results (as above.)

The new device is capable of gigabit speeds. This is my first encounter with a gigabit networking device. I had assumed that it would auto-sense and drop back to 100Mbit, like the 100Mbit card can drop back to 10, but I don't know now?

Does anyone know if these symptoms indicate I was wrong about that compatibility?