Vista - cmd window, can't run any programs, access is denied

Hi,

I think I've done something to my computer, but I'm not sure what. I
can no longer execute any command line programs from a cmd window. I
get "Access is denied". That applies to find, more, help, etc.,
everything, including cmd itself, which should open up another window.

I have no trouble starting a cmd window from the Start menu, though.
And if I type "more" in the Start menu I get a window with more.com in
the title.

It's just that nothing works from a cmd window itself.

Desperate for any ideas. Thanks.

rd