All of the Slackware init files are located in /etc/rc.d. Go there and edit rc.modules; comment out whatever module is giving the error. Chances are you have updated your kernel from the one you selected at install and now it is looking for a module you no longer have/need. It's also a good idea to run depmod -a to keep module dependencies up-to-date (as gore suggested) but slack should do this on every boot (look near the top of the aforementioned rc.modules file)