I had the same problem. I'm using a Dell that never gives the option to enter BIOS. I installed slack by:
1 - downloading RAWRITE.EXE from ftp.slackware.com
2 - downloading one bootdisk and two rootdisk images (bare.i bootdisk, install.1 and install.2 rootdisks)
3 - from a DOS prompt, using RAWRITE to burn the images to three floppies. NOTE: I "borrowed" some used floppies from work. I had to discard and remake two of them because they had bad sectors or something and gave read errors when I went to the next step. Floppies seem to get less reliable every year.
4 - reboot your computer with the first disk in the drive. Insert the second disk when prompted, then the third.
5 - login as root and you're ready to install