Focus Issue

I have been using Vista ultimate x64 now for about a month and I am largely
very happy with it.

I have a very strange and annoying bug/feature however.

Every now and again (frequency varies seconds/minutes/hours), I find that
having switched away from 1 application that I can no longer click the original
application (in the background) to bring it back to main focus.

Somethimes this happens even when there is apparently no application in front
of the app I want to use..

The only way around this is to click the tsakbar button of the desired app
or Alt-Tab back to it.

Can anyone tell me why I might be experiencing this strange side effect?

I am multiposting this to security as it seems an aweful lot like when the
UAC kicks in to ask me a question except that the screen does not change
visually.

Thanks in advance for any information

--
Rory