Most people are going to recommend you use grub. grub is just plain easier (in MOST cases).
Boot a linux live cd (knoppix is nice), and use gparted/parted on the hard drive to see if the hidden partition is at least a marked partition. If it is, you don't have to worry about it. The next likely problem is disk space. Your Win7 install is probably taking up the whole hard drive, and I don't know enough about windows to say if Win7 uses the same ntfs format as XP/Vista. if it does, you can probably resize the win7 partition safely. if it doesn't, I'm not sure if it can be resized.
Once you've made it that far, you should be able to install slack to the new free space (not used slack in a long long time, so don't take that at full face value). When you get to configuring the bootloader, don't have it installed into the MBR, but rather, into the beginning of the partition. When that's done, you can use a dd command to copy the bootsector to a file, and edit your boot.ini for windows to have an entry listing your linux install. This is how I did my first dual booting win2k, anyways.
That's just a light overview of what you're going to be doing. I'd advise a step '0' before doing any of that where you get a sufficiently large external hard drive to do a dd of the whole hard drive as it is right now, that way you can do a true full restore if you mung anything up.