Slackware 8.0 shipped unconfigured kernel sources, so they don't match your running kernel. This is why you're getting unresolved symbols; your kernel sources need to be configured to match your running kernel.
To fix this, you can either configure and recompile a new kernel, or find a .config that matches the Slackware 8.0 default kernel config, and update your kernel sources that way. You can probably find the .config through google - I seem to remember someone posting one here recently, although I can't seem to find it on a cursory search.
I hope this helps...