Did you burn the CD as an image or data disc? What program are you using? Alcohol 120% will write CD images, and I'm pretty sure Ahead Nero will too if you click "disc image or saved project" and select the *.ISO.
You can configure a dual boot, first you will need to get Partition Magic, which costs money. I havent found a program that does the same, but others may have. Then I resized my main partition, and it left some unpartitioned space. I would use about 10gb. Then all I had to do (with Ubuntu) was boot from the CD and install on the new partition. It installed GRUB bootloader over the MBR, and it automatically identified Windows XP. Then after Ubuntu was finished installing, I had to change hard drive to "large" in bios, and it all worked just fine. I'm sure Slackware would do the same, maybe someone can tell you for sure.