Windows don't come to the top

Occasionally, my Windows 7 (x64 Ultimate) gets in a state where newly opened windows open behind existing windows. If I click the icon of an open task in the taskbar, then the taskbar icon blinks, but nothing else changes. The clicked application doesn't come to the top.

This makes it difficult to surface windows that have become covered by other windows, as I have to minimise the obscuring windows until I find the one I'm trying to use. Clicking that brings it to focus, and the taskbar icon stops flashing. But then I have the problem or re-surfacing the windows that I minimised... eventually I reboot in frustration.

Does anyone have any idea what might be causing this?

You should be able to minimize windows you are not looking at by clicking it`s open icon on the taskbar, this will shrink it down, do it til you get to the window you want. But I`m not sure why when you click on an open programs icon on the taskbar it doesn`t just bring up that open window.

Hello.. i just use tabs.. not sure why different windows of a browser needed.. but.. does seem strange.. as each window would give you a new icon in task bar to click on to get into . sorry i offer no advice on how to fix.. should not be happening as above poster stated..

Not sure if he`s actually talking about multiple browser windows, just open windows in general.

Yes. as I never have an issue to access a tab or a new window page of a browser

Sorry if I wasn't clear, these are Windows applications that I'm describing. Each one has its own icon in the taskbar (or sometimes in the System Area).
This is a rare occurrance (maybe once every two months or so) and I suspect it is something running on my PC that is causing the problem. However, closing all the open applications doesn't fix the issue. It must be something that runs with no visible UI. Like a service, for example. I'm not implying that it is something malicious, sneakily hiding on my system.

I was hoping that someone would say "I had that, and it was caused by X", but this is really a stab in the dark; I don't really expect to get an answer as easily as this...