Hello!

1.) You should save your mbr with the dd command,
because WindowsNT overwrite it during the installation.

dd if=/dev/wd0d of=/tmp/mbr.bin bs=512 count=1

This should save your mbr from wd0 to the file /tmp/mbr.bin

2.) You should have a boot disk or cd to boot your NetBSD 
after the WindowsNT installation.

You can now easy restore your old mbr and boot your NetBSD.
Now you have to use a bootmanager for example the NetBSD 
internal or other bootmanagers like grub.

Hope that helps - cheers,
Martin Wittwer

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