Don't necessarily count on this but first off, did you set your bios to read off the cd first?
If so, installing slackware should be a snap. Just put in the cd after changing your boot sequence so cdrom comes before harddrive and floppy. Pop the cd in and you should be on your way to slackware goodness. Mine installed first time, granted i dont have redhat
like you do but it should still setup.
If and when your cd starts up and it starts prompting you you'll have to partition for more space...blahblahblah.... then up will come lilo config eventually after install. When it asks you if you what setup you want ie. simple, expert and so on choose expert.
And do the options in order.
Add your bootable partitions in linux, both red hat and slackware boot partitions and Wintendo er Windows XP and configure, follow rest of set up and you should be off to slackware goodness.