slow networking nforce 4

I've installed Vista Beta 2 (5384) x86 on my Pentium4 + nForce 4 Intel Edition machine. I use the integrated Ethernet on the nForce 4 to connect to a big coporate network and I connect to the internet through a proxy on this network. I have no other way to connect to the internet, direct connection is impossible on this network. Vista tells me that I am not connected to the internet (local only connection), however I could activate Windows and IE can load some sites, some instantly, most are annoyingly slow and some don't load at all. Windows Update takes some 15 minutes to download 1% of a 5.5 Megs download. On XP I usually do 200-500KB/sec. I'm using the network drivers that come with Vista, there are no nVidia Vista network drivers.