The installation of virtualbox compiles a kernel module. However, Slackware kernel header sources have been pre-configured to create modules for the SMP kernel. You installed and are running the non-SMP kernel.
It is easy to reconfigure the kernel sources in /usr/src/linux so that they will create modules for your non-SMP kernel instead. Read http://slackware.osuosl.org/slackwar...sdk/README.TXT
and check the directory in which that README is located.
After modifying the kernel sources, you will have to run the virtualbox installation again to produce a working kernel module.