Hard drive activity. How to swithc defrag OFF?

Hi.

I have a newly built home PC running Vista Ultimate. It's a
reasonably high spec, core 2 6600, 2 gigm ram and 8800gtx graphics.

So I finally installed Battlefield2142 and tried to have a game. My
god, so slow. Why?

Well the HDD was chuntering away. All utilities are switched off
including antivirus. Nothing going on the net. Check defrag. Last
activated 20 minutes ago. OK, how do I switch the damn thing OFF! It
has no controls but a setting to make it run on a schedule. (Which I
have set for 3am!)

Any clues please folks?