I boot WinXP, Mandrake and Fedora. Windows is on /dev/hda1, Mandrake is on /dev/hda9, and Fedora is on /dev/hda8.
Like this: (my lilo.conf)
append="devfs=mount resume=/dev/hdb1 splash=silent idebus=66"
append="devfs=mount acpi=ht resume=/dev/hdb1 idebus=66"
Windows is the default because I'm not the only one here that uses the computer.
In your case, it depends on what drive Windows is on and what drive Slack is on, and then on which partition your are booting Slack from.
You could probably use my lilo as an example for editing yours, adding the Windows entries as they are in mine, but change the drive assingment. In my case, Windows is on my primary master, hda, partition 1 (hda1)If your Windows drive is the primary slave it would be hdb, and likely partition 1 (hdb1). Gee, can Windows even boot from other than primary master if it was originally installed as such?
Save a copy of your lilo.conf before you make changes. If things go wrong you can boot with a linux boot disk and copy your old lilo.conf back then run as root /sbin/lilo. Then you should be back to square 1.
Above all, don't forget to run /sbin/lilo as root after any changes to lilo.conf.
Then you can re-boot.
Hope I make some sense here. Would love to hear back from you the results.
Running Mandrake 10 and Fedora Core 1