I noticed that when rendering the disk activity is high. Since RAM is so cheap any more, if you have 4 GB ram, I recommend making a ramdisk and rendering to that device.
See http://www.mydigitallife.info/2007/05/27/free-ramdisk-for-windows-vista-xp-2000-and-2003-server/ for a setup article.
I really can't quantify the speedup, but if you have a slow disk, it HAS to be faster.