Thank you everyone for your replies.

I discovered that my old rig actually had a slightly better CPU (i7-2600) so the new rig wasn't 'better in every way', as I originally thought.

I found that doing a clean boot seemed to stop the excessive CPU use but, as I was also faced with an interminable number of BSOD issues, I decided to upgrade my CPU (to an i7-3770K) and do a fresh install of Windows 7.

Needless to say, that's solved all my problems. I was more diligent with the amount of programs I installed, and did it in a more piecemeal fashion this time around. CPU use sits at 0-2% now, as it ought to.

Thanks again.