There have been numerous threads about this subject (and undoubtedly will be many more) and their gist is: install Linux wherever you deem appropriate and install the bootloader to the MBR (master boot record). I don't know about Slack, but many distros automatically include an entry for windows in the bootloader.
If it doesn't, read up on Grub (or LILO if you must).
If you have windows XP it is also possible to have windows ask you what to boot. In this case you should install the linux bootloader to the drive that contains /boot. I recently received some very detailed info on this in a thread which you can read here