Computer "locking"

[Caribeing]

I have Vista Windows, my computer has what
I can only explain as "locking", When I am
on Google etc or other sites, my screen goes
blank and returns to "insert Password", when I do this it brings back to my previous screen, hope this explains my query please help!

[ACtheTROLL]

are you an administrator or a user?

It sounds like UAC is intervening
but why when on google I can't think (it's late)

look here
carefuly follow the instructions in the
Turning off UAC section - and see if it stops happening.
http://technet2.microsoft.com/WindowsVista/en/ library/0d75f774-8514-4c9e-ac08-4c21f5c6c2d910 33.mspx?mfr=true