You could try to reinstall, but you should:
1) boot the machine
2) run fdisk and create:
32M swap (type 82)
all the rest for Linux ext2 (type 83)
3) Run 'setup' directly
4) Follow the steps
If this doesn't work, try to clean CDROM, verify cables. I had to (try to) reinstall Slack 8 for 5 or 6 times, because the installation hanged, because my IDE cables are broken...