If you navigate through the viewport( in perspectief view press and hold left alt and with the rightmouse button holded scroll the mouse), does it respond properly?so no delays and such.
because my first ques is that your system isn't up to par.
you need something like a 2-3 ghz processor a pretty good videocart , at least 1024 mb ram .

do you have the latest drivers of everything , no virusses etc.?

anyhow good luck