And just in case it's the 'file types changing' problem

Yummy_Mummy23, have a look at the extensions of your files...have they changed from whatever they were (mp3, avi etc) to zip or exe? If that's happened, you probably have a worm. If the extensions haven't changed, ignore this

Update your AV & do a scan, see if it finds a worm (Delf.ATB). Or you can 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 a few people have had this problem...AVG has fixed things for some

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