Computer sluggishness and semi-freezing up…

Can anyone shed some light on this issue? I will have one window open with no other programs running and some times my computer will freeze up. My mouse cursor still moves, but I cannot click on anything to shut the window. So, I do this, , and as my task manager pops up, it unfreezes my open window, and I can minimize it or close it. Has anyone ever heard of this problem? Any comments or advice is welcome. Thanks.