Defrag won’t go below 23%

I’m suffering from a wicked slowdown with my computer. It seems to be having a hard time acessing the hard disk. I get a lot of hangs that last for several minutes. Defag won’t go below 11%for “total fragmentation” and 23% for “file fragmentation”. I’ve used both norton system-works defrag and windows xp defrag, with equal results. I use Webroot spyware and norton anti-virus. I can’t find any other reasons for the major slowdown………help.