Are you running Slackware's "huge.s" or "generic.s" kernel? Not the SMP version but the uni-processor kernel? If that is the case, then you need to "fix up" your kernel source by looking at
/extra/linux-2.6.24.5-nosmp-sdk/ and applying the information you find there. By default, compiling additional kernel modules like ndiswrapper, will create a module that works only with the "smp" versions of the Slackware kernels. If you run a non-smp kernel then you need to apply the above patch and then re-compile your ndiswrapper.
Eric