The Task B3 in the last link of my signature is based on using any Live CD to rescue Lilo of an unbootable Linux.
If you have a working Linux then just use it to rescue the unbootable Linux using the the same commands.
Basically you mount the Slackware partition in PCLinuxOS, change root to it so that you are physically inside Slackware and use its own command "lilo -b /dev/hda" to restore Lilo iknto the MBR of hda. Once inside Slackware you can actually run "liloconfig" again which is the program the installer used to implement Lilo. The "lilo -b /dev/hda" will report to you any error if Lilo cannot be restored.
Once your Slackware boots you should find Grub will be overwritten by Lilo. That is alright. All you need to do is to choose which boot loader to take over the MBR and make all the other distros "chainloadable" by this boot loader.
If you want PCLinuxOS booted by Slackware's Lilo then use SLackware to rescue PCLInuxOS by restoring Grub
inside PCLinuxOS root partition. Say if PCLinuxOS root partition is in /dev/hda6 then Lilo can boot it if you add these two lines in Slackware /etc/lilo.conf
Code:
other=/dev/hda6
label=PCLinuxOS@hda6
Consult Task D in the last link of my siganture of making a Linux chainloadable.