Yeh it becomes really slow when you have lots of downloaded junk on there. THings lke kazaa and imesh dont help it at all, it might pay off just to get an antivirus and check it out that might help but delete most of the things you dont need, maby defragment it or format it and that might help.
You can defragement it through C:/Windows and it will be in there or maby load up your windows again.