i'd always recommend doing it this way, two oses two drives. nice.
so...
remove the slack drive completely. configure the windows drive as a master, and install windows. make sure it's all work and then revert back to slack as master. boot to slackware and edit your bootloader config. here you need basically lie to windows, so when it boots it thinks it is the master drive as it was when it was set up, even though it's physically running as a slave. under grub it's just a case of adding this:
Code:
title WinXP
map (hd0) (hd1)
map (hd1) (hd0)
rootnoverify(hd1,0)
chainloader +1
to make a new entry for windows. should be that easy.