pdfs disappear from desktop after restart

[emptyl]

I've run into an odd problem in the past couple of months - every time I restart my computer, all pdfs (but no other file types or folders) are erased from my desktop. I'm baffled as to the cause, and unfortunately it has led to me losing a large quantity of very important data whenever I forget to file pdfs away. They can't be retrieved through the trash or by performing a comprehensive search. They don't seem to be hidden, or visible in Finder in any form. Any ideas? I'm running OS 10.6.8 from a MacBook Pro. Thanks in advance for your advice!