IPSec and Juniper

Hi,
I would like to know if it is possible to configure IPSec between a
Windows 2003 server running ISA and a Juniper M20 router.
The Windows 2003 server is in my control while the router belongs to a
partner company, I have tried configuring through secpol.msc but not
made much progress.
I was advised by the administrator of the router that the IPSec I
create should support fragmentation of IP Packets before encryption
(Pre-Fragmentation). How do I do this in Win2K3.

Thanks for your help,
Pranav