PC: "Could not lock" error in Win

1
I'm a relative newcomer to psychtoolbox so apologies if there is an
obvious answer, but when I try to run some of the demos (and my own
code) on a Dell Optiplex GX280 (running 2K), I get 'could not lock'
errors. Eg...

MovieDemo aborts with
??? SCREENDFillOval: Could not lock device context for drawing.

NoiseDemo aborts with
??? PutImage: Could not lock the offscreen surface for writing.

Same code (and my own code) works fine on a Sony Vaio. Any thoughts?

Thanks,

George

--
-------------------------------------------
Professor George Mather
Psychology Department
University of Sussex
Falmer, Brighton BN1 9QG
Tel: 01273 678342
Web: www.lifesci.sussex.ac.uk/Home/George_Mather