Can't import to Movie Maker

Hi. I'm new to the forum and have a problem. I had a number of videos on my computer given to me by a friend who took the videos with their camera. I merged and converted the MP4 files into one .wmv file but I can't import it to Windows Movie Maker. When I try I get a message saying the file "could not be imported". I had previously converted and imported one of the smaller files without a problem but can't import the merged files. The software I used to convert and merge is called Hamster Free Video Converter. Is there a size limit on imported files? Any ideas would be appreciated.

Andy