First guess: you run a non-SMP kernel but the ndiswrapper module is built for a SMP kernel?
Your kernel version (2.6.29.6) indicates the non-SMP kernel (the Slackware SMP kernel has "2.6.29.6-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-2.6.29.6-nosmp-sdk/ to your kernel source directory (read the README in that directory for a better understanding).
Eric