New task windows will not open unless current window is closed/minimized

Suddenly (since yesterday) I can not open a new window from the task bar unless I close or minimize the window I am currently in. For instance, if I am in Photoshop or on a web page I can not open Outlook unless I minimize or close the program I am in. Same thing if I'm composing an email, I can't go to outlook or open any other program unless I minimize what I am working on. This occurs also in IE or FF. The programs are on the task bar and are lit up. I believe they are actually open but aren't "coming to the front".

Hope my non tech explanation is clear. This is very frustrating and time consuming to deal with.

I think something got corrupted. It's sort of a mix between "always on top" and "active window". I would try doing a system restore to before yesterday.

Thanks. I think it may be a memory issue. I used ccleaner, manually deleted a bunch of cache files etc. After restarting my computer every thing seems to be OK.

Still would like a definitive answer as to what causes this and how to prevent it.