Hi! How did you install Slackware in the first place? Do you have disk 1 and disk 2?
If you have them,
Insert disk 2 into the cd-rom drive and boot that up.
Once login mount your root partition to /mnt/hd
for example mount /dev/hda2 /mnt/hd
Then chroot /mnt/hd (I haven't use this command for a long time, it may be the partition label such as /dev/hdXY)
Go to /etc/lilo.conf and edit it.
Add your windoze installation to it by adding the following line
other = /dev/hdXY
label = Windoze
to the bottom of the file.
Then issue the command lilo ( this will reinstall lilo to MBR).
I suggest using lilo as your bootloader for multiboot situation. Your opinion may defer.