you could use mtu discovery protocol or mss clamping from ipnat. And this is 
not a windows problem. It's normal if two peers are directly connected to a 
1500 MTU network and they are not aware of the presence of a tunnel with some 
smaller MTU between them.

M

On Thursday 03 October 2002 23:35, Brian Buhrow wrote:
> hello folks.  Under cisco IOS, there is a command you can use, when
> configuring gre tunnels, for allowing the mtu size of the tunnel itself to
> be 1500 bytes.  this causes encapsulated packets to be fragmented and
> reassembled at each end of the tunnel, but allows brain-damaged Windows
> machines to use the tunnels without user intervention.
> 	Can the gre(4) or ipip(4) drivers perform this
> trick and thus provide a 1500 byte mtu through the tunnel?
> If so, how do I configure it?
> -thanks
> -Brian