hmm, windows vista got window defender. what does it do? i already got avg, do i need to disable one of them to free more resources?

i'm having a prob with it. Every once in a while (ie 10 mins, sometimes 30 mins) my hard disk activity jumps to an insane amount to the point everything slows down. Movie becomes still frames pictures.. This happens when i have new files on the computer (ie download stuff from torrents). Is this normal or is it because some program is running? checked the performance monitor and it shows my harddisk activity at 100% and it's usually one file that's being access the most. KAV does not show any antivirus scan running and there's no virus detected on the pc.