you need to change the boot order in your BIOS. on Slackware installation, if you partition your drive properly, mandy and slack can run side-by-side. which i recommend, especially for us newbs
on boot, hit either F1, DEL, or whatever key gets you into your bios, from there change the boot order to boot from a CD before a hard drive. It's very easy to figure out...
reboot with the CD in and you're good to go