Well I have a couple things you can check. First thing to do would be to boot into safe mode and check for any duplicate drivers in anything. (Safe mode shows installed drivers, Normal mode shows in use drivers.)

Anouther thing to check is to make sure both Windoze are using the same driver for the NIC card.

And last is to check Speed Guide's website. This information is geared toward cable/DSL modems, but it has a lot of useful information on utilizing network bandwith in Windoze.

Hope this helps.