I reckon it sounds like you're infected. A few people have had this happen lately...there's probably a new virus on the loose.
File types should not change. LW does not change file types. Are you scanning files with your AV before you open them?

Scan your computer with a good, updated AV. It wouldn't hurt to scan for spy/adware as well. There are links to programs within the thread below.

If your scans don't find a virus/trojan/worm etc, I'd suggest you go to a help site for this. You may need to post a HijackThis log & we can't read those. See point 5 here for links
WARNING: Viruses on the network you should be aware of
*(Norbiesworld is no longer providing help, so don't go there)

Or try one of these
http://forums.spywareinfo.com/index.php?showtopic=79038
spywarewarrior.com
http://www.bleepingcomputer.com/forums/