help with cpu upgrade

Right I was upgrading my pc, put in 2gb ram, turned it on everything fine.
put a new graphics card in turned it on – fine
put new hard drive in- fine all formatted ok
changed the cpu….
Fisrt of all there was 1 long beep, 2 short beeps, which is apparently a video error so i removed the new graphics card, started it up same beeps again, went to the mobo manufacturers site they told me to clear my cmos, which i did. Started it up again, same beeps.
Decided to put the old cpu back in, and now it starts normal beep. but it doesnt start booting it just sits there with the name of my mobo on the screen.

Mobo asus m2npv-mx

help?!