Where'd my pics go?

I have an external hard drive for picture storage, but recently when I went to view the contents(in thumbnail view), while almost all of the orange boxes(Jpeg) became thumbnails, some did not. When I tried to view them with Windows Picture and Fax viewer, all I got was "no preview available". Trying to Edit in Paint gives me "unsupported filetype". So then, my questions are these:

Is there any way I can recover these files?
Do you know what may have caused this?
Is there something I can do to prevent this from happening?

Its a known issue with Windows: You need to remove the files and folders from your /system/ and /system32/ folders as they are where illegal downloads and viruses and other spyware download to and cause conflicts with your picture viewer

Yeah, its a very common defect Microsoft never cared to fix themselves because it is as simple as deleting those folders.