First guess: you run a non-SMP kernel but the ndiswrapper module is built for a SMP kernel?
Your kernel version (18.104.22.168) indicates the non-SMP kernel (the Slackware SMP kernel has "22.214.171.124-smp" as it's version). However, the Slackware kernel source package is pre-configured by default to build kernel modules for a SMP kernel.
You have two options: (1) start running a SMP kernel, or (2) apply the patch script found in /extra/linux-126.96.36.199-nosmp-sdk/
to your kernel source directory (read the README in that directory for a better understanding).