From the ndiswrapper wiki:
"If you get "unresolved symbols", you may be using old kernel; upgrade the kernel and recompile ndiswrapper and reinstall. If you get 'loadndisdriver' failed, then you may have older version of ndiswrapper module in kernel modules directory. See Uninstall to remove all ndiswrapper files and reinistall again. See also FAQ for more details on common issues with loading ndiswrapper module."
Did you install ndiswrapper from source or use a Slackware package? If from source, was your Slackware install new, i.e., was 2.4.33 the kernel in place when you compiled & installed ndiswrapper?