PC was fine with 32bit going slow on 64bit! HELP

Hi I have a fairly new (late 2011) PC specs are i5 k processor,Intel H67 Mobo,12 gig ram,Seagate Caviar Black and Blue drives,with Focusrite Saffire DSP24 audio,Matrox G550 LP PCie card.Cubase C6.

This PC was flying along under Win7 32 bit.I realised quite quickly I need to go to 64 bit so upgraded the memory using the same type 1333 as already in there (originally 4 gig). So I did a clean install of Win 7 ultimate 64 bit.Install was fine then loaded the latest drivers.No clashes or warning came up during install.

But the PC now is so sluggish and slow its like a different machine.Even just on the desktop with no progammes running there is a noticeable delay in opening the start menu and if I was moving an open window around you get a ghosting effect (looks like multiple verions of the same window dragging behind).

On using Cubase C6,the response is slow and opening the menu options you can see a noticeable delay.Cubase also takes longer to load and close down.So you get the picture.Am not a happy bunny right now.

When looking at the windows experience index it gives very good rating for the processor and memory but very low (1) for the Grapphics - in Cubase I have noticed the video player says ‘unable to play video’

But the graphics card isnt that bad? I have run DPC latency checker and it has given me spikes to indicate there is a problem but where?

Has anyone any ideas of how to assess or troubeshoot this?