Your file extensions probably changed because your computer's infected...sounds like you have a worm. Try AVG Free AV...that should find it.
http://free.grisoft.com/doc/2/lng/us/tpl/v5

Download AVG, update it, scan your computer.
Search the forum & you'll see other people have had this problem...AVG has fixed things for quite a few

If you need more help, try one of the sites below.

http://www.spywarewarrior.com/index.php

http://www.castlecops.com/forums.html

http://forums.spywareinfo.com/index.php?showtopic=79038


Try the advice here about file recovery. Some people have had good results after using Restoration.
Deleting files from LW library - Don't!! File recovery advice