Second drive dual booting
I have Mandrake 8.0 and am basing this answer on that install process OK?
Install your second drive and then install 8.1 onto that drive (it should be appearing as hdb - hda is your first disc with windows on).
Once you get to the LILO (Bootloader) part of the install select hda as the place to put LILO and not the MBR (master boot record) of hdb - this should mean that when you boot hda(as master) will run LILO and give you the option to boot into Linux or Windows.
Hope this helps you - but don't forget to go to the Mndrake-Linux site itself as they have an extensive help forum at MandrakeUser.org and other resources for you to call on.