Black screen on startup - Dell Vostro 470

Hello,

I was wondering if anyone would have some sound advice for me on this problem. I have a Dell Vostro 470, with Intel I5-3450 @ 3.10 GHZ, Nvidia Geforce GT 620 video card and 6 Gig of ram. When I start the computer, it begins normal startup, then just before the windows desktop appears...I get a black screen that shows only the mouse pointer. Sometimes it will show the mouse pointer and the taskbar but no icons. From there, I cannot shut it down unless I push the power button. Im using Windows 7 64 bit version. Usually it will startup normally afterwards that but not always. At first I thought this just happened when the computer is cold, but it happens either way. And sometimes it just starts normal. I did a full virus scan (using Norton 360) and no issues there. I used malwarebytes and it shows clean. I switched monitors and same problem and I also switched out the video card with another and still have the same problem. Any help on this would be much appreciated.

Cheers

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