background won't stick

Computer is operating on Windows XP connected to a server with full
admin rights to the local machine.


If we change the background it sticks for a while but then changes by
itself and sort of tiles the image, but not exactly. It divides the
large full screen picture and puts the bottom right corner of the
picture in the top left corner of the screen and the top left part of
the picture in the lower right corner of the machine.


For example we put the coffee bean back ground on and it does fine
for
a while. Later it will have the moon back ground image, divided as
described above.

any suggestions would be appreciated.