With so little info on the actual cause of error, I will go ahead and speculate:
If you're importing large video files into WLMM from Windows Explorer, you must remember that it will always try to read the file's index (which is used to make future searches faster). WLMM, to my knowledge, has to go through the whole file to get its properties like video and audio codec, bit rates, key frames, type of frames, resolution, aspect ratio etc. and the bigger these files are, the longer it will take.
If WLMM freezes a lot for you when importing that kind of files, don't try to do anything because it will surely crash. Be patient and wait until WLMM does its thing.