Can't change Windows Aero desktop/theme

Most of the Windows Aeros functions are OK - glass, flip etc. but I can't change my desktop to anything other than a solid colour (no wall paper/ pictures etc.)

Also, if I change the Theme to Vista, the desktop defaults to black.

I can change the deskop from within Picasa, but the next time I re-boot it will have reverted to black. I thought the problem may lie with Picasa, because when I fire it up, I get message saying the colours have been changed to Vista basic.

Very frustrating. Although only cosmetic, I wonder whether this lack of function will have implications elsewhere.

Any ideas?

My PC spec is
ACPI+86
NVIDIA Geoforce 7300SE/7200 GS
Intel(R)Core TM2 CPU 6420
IGB RAM
Base score 3

not sure of this thing.becoz my pc has no such prblm.any way see

http://www.yourpcpartner.com/solutio...und.htm?page=2