Sure, thanks for your reply
I had MSN 7.5 and Messenger Plus! 3 (all latest versions).
I then installed Windows Live Messenger (when it became final) which replaced MSN 7.5
I then installed the latest version of Messenger Plus! Live which seemed to work, so I then uninstalled the old Messenger Plus! 3.
I didn't use any kind of cleaner/optimizer software/antivirus software.
I never touched any files in that folder, and I have only 6 files in there now - these are sounds I've been sent since I lost them all.
This has happened on two computers that I know of, and maybe many more!