Ok, lets start from beginning, so you have flashed your bios with latest f/w, rebooted, loaded opimitised defaults, saved and rebooted.(you should not get checksum errors after this step)
Then you went about installing xp, installed latest nforce drivers, rebooted, but the marvel does not get installed.
did you go thru' the m/c manual or cd, are their any special ways to install the marvel driver or any special setting in the bios which is required.
My suggestion, just a try :
Uninstall the (marvel+normal lan) from your DM(device manager), reboot, get into your bios, disable (marvel+normal) lan, reboot, get into DM do you still see the marvel or normal lan, if yes stop and post back,
If not, then uninstall the nforce drivers, reboot.
Reinstall the nforce drivers but this time dont install the nvidia firewall during setup options, just the ide, sm and ethernet drivers. Now reboot, get into bios and enable just the marvel lan, see if the marvel works. else -
Reinstall the nforce drivers but this time dont install the nvidia firewall and ethernet driver during setup options, just install the ide and sm drivers. Now reboot, get into bios and enable just the marvel lan, and let windows install the driver for marvel lan, see if the marvel works.
Remember, we are disabling the other normal lan to check if it conflicts with marvel in anyway. also as i said before, do go thru' the manual or asus setup cd, maybe we are missing something.
Also, you can pm TE members having the same board to know if they have same issues.