I hope you made a bootdisk when installing slackware?
First lesson of Linux, ALWAYS make a bootdisk.
And as for the problem with the kernel-panic, I had the same problem as you and this is what I did:
1. Disable PnP-OS in BIOS, this should eliminate the "..is not SMP safe" error.
2. Boot using the install-cd, enter cfdisk and check that everything seems ok.
3. Do a file-systems-ceck 'fsck [your-root-partition]' (i.e /dev/hda1)
When i did this fsck found a whole bunch of problems with the root-partition and repaired them (bad blocks and stuff). After that i rebooted and it worked like a charm again =)