Yeah, movie maker should be able to handle large files perfectly fine (I have raw DV-AVI files that are 9 GBs). Are there any particular errors, did you check the event viewer? If there's any kind of file that WMM should work with it's WMVs. It could be that the WMV file you're trying to edit falls outside of some specification for WMM to handle (like 6 channel audio or something).