I'd recommend converting the movies to a format like MP4 (which you can do in Windows Live MovieMaker).
WMVs are just too iffy of a format for using as source video in Premiere Elements, IMHO.
BTW, you are adding these files using Add Media/From Files and Folders, right? You do not add media using File/Open.
In any event, a conversion to a more easily editable format will save you a lot of heartache.