Hi tesa
,
Well for a quick boot it quick,
1) Configure on your bios booting from CDROM
2) Put your disk #1 on your dcrom drive
3) when starting slack follow the instructions, I cannot remember well but should be something like this just red the the example and put it:
root=/dev/hda2 ro
NOTE: hda2 assuming that hda2 is your /
if is a ata disk should be sda2