Unless your computer is very very old, the current Slackware should run on it.
I ran Slackware 12.1 on a IBM PC 300 with 385 MB RAM (one of the original Pentiums). It likely would not run KDE 4.x, but it was okay, if a little slow, with KDE 3.5, and acceptable with Fluxbox. It would not play video, but would play audio. I was using it as a file server, so I didn't care about the GUI all that much--I just wanted the hard drives spinning.
I agree with GVroovman. You need to format the MBR.
Set the BIOS so that the boot from CD option comes before the boot from hard drive option, and you should be able to boot to the Slack install CDs; Slack gives you the option of putting LILO (the LInux boot LOader) on the MBR.
Here's good expanded explanation of how to format the MBR using dd.