Couple of things to check if you haven't done so:

Make sure the correct adapter is listed under IP configuration screen of the ARM, since you have 2 ethernet adapters. Also, the box at the bottom of the IP configuration should be checked.

Make sure that you have unplugged the RIO unit, waited a bit, then restarted the unit. Try with and without auto configuration enabled on the ARM IP configuration screen. I usually deleted the old units showing up in ARM prior to this step.

View network connections under Windows XP. Determine if ethernet adapter you are using has an IP address. You might try to let WIndows XP software bridge the 2 ethernet connections, then go to the IP configuration in ARM and use the bridge to serve up the Rios.

I had some annoying issues once I went to Windows XP and finally had to get rid of the multiple network adapters in my server to create stability. I currently have one ethernet card in the music server which connects to a cheap ($20) Linksys switch. Connected to the switch are other computers in the same room, a Rio, and a 2 wire router with HPNA support for the rest of the computers and Rios in my house. Its very stable now..

Good luck.