Did you build the ndiswrapper module yourself or did you install a binary? If the latter then you should go to http://ndiswrapper.sourceforge.net
and get the source code and build the module against your kernel sources.
Now after saying all that I would really recommend you use the open source rt2500
driver instead of ndiswrapper + rt2500.sys. In Windows and in Linux using ndiswrapper my wireless connection is pathetic. It dropps out after 1 - 5 min and needs restarted constantly. Since I switched to open source rt2500 I have a good strong uninterrupted connection!