Missing graphics files

This morning all my graphics appeared to be missing. In a few cases an icon appeared in Windows Explorer without a preview and when I tried opening the file, the system said the file did not exist. In most cases, the directory containing the files appeared totally empty and a check of disc space indicated that there was more much more free space than the day before. After rebooting the computer everything went back to normal so it is not really a problem at the moment. My question: is there some malware than can do this? Can specific types files, such as all graphics, be targeted.

With very few exceptions, I keep all my data files on an external drive. I've had too many computer crashes over the years and do not trust having my data on the same disc as the software. I have literally tens of thousands of graphics files on the external disc. Should I backup to another disc that is not always plugged into the computer?

I use Firefox as my browser and I have the WOT extension on it. This week I've been getting a lot of messages about sites being untrustworthy even though I was only downloading OpenOffice extensions. Could these two issues be related?