Map drives across VPN fails

Creating a VPN connection from my Vista Ultimate RTM PC to any of my 10
different destination networks works fine.

Once connected, I can ping, RDP etc to remote servers across the pptp tunnel
fine.

However, I cannot map drives. I can with exactly the same configuration
from an XP SP2 PC, so there are no "network" issues involved here - it must
be purely related to changes in Vista.

I have tried all the usual suspect like disabling the firewall, disabling
autotuning etc, but with no success. I have tried mapping to IP address
rather than netbios name, providing credentials using the net use command -
but everything is met with system error 53. I can't find any local policy
settings that may be causing the problem either.

I ran some network traces while attempting to map drives, and noticed that
Vista was actually, bizzarely, attempting an http connection! Stopping the
Vista Web Client service seems to resolve this somewhat, and now I get system
error 67.

Network traces now show a TCP checksum error in response to netbios-ssn
traffic.

There must be some change in the Vista TCP stack that is causing this, but I
can't figure out what - In fact I am convinced it is a bug.

I see there are other posts on this exact subject, but am hoping from an MS
response as a Technet Plus subscriber..