Usually, the higher preview resolution you go, the more sluggish you preview gets. So the lower the preview resolution you go, the smoother your preview is, but at the disadvantage of rotten resolution. But that does not have any effect on your rendered project. Just your editing preview.
But now that you have made a good video, you see the difference as your final rendered video normally looks great.
I bounce between high resolution ( I do not have the HD options as I am 32 bit) and normal depending on how much movement I have put in the project. Normally I put in a LOT of movement objects.
Windows 8 Pro 64 bit
CyberLink PowerDirector 10 Tutorials
PDtoots PowerDirector Tutorials
When you are asked to provide a DXDIAG you go the following link and do part "B". Your posted specs are NOT what we are looking for as they tell us nothing. The specs on the box of your computer mean nothing. The DXDIAG shows us how your computer is configured as it runs.