how much free hard disk d u have? if it's less than 10%, it's no wonder

Windows Vista constantly defrags your hdd whenever there's no activity in your windows

yes, u don't.

u can even defrag with just 1% free hard disk space, but it will take ages, especially if you have large chunks of files

the more space it has, the faster the initial defragment process will be

plus, it'd help if you stop torrenting when you defrag