JPEG files disappearing

Recently downloaded and worked/printed JPEG files imported from an SD card
reader using Nikon Picture Perfect. The card is a PNY 4 Gig and the camera
is a Nikon D80. The PC is running XP home SP2 on a Quadcore PC. This
process has been done many times in the past with no problem. A few days
later some of the folders that were created were still there but all the
pictures that were once in the folder are now gone. A folder that once read
3+ Gigs of JPEG files now read 0 bytes. Date created, modify dates and times
are suddenly identical. This happened to about 4 folders of picutres. HELP?