The most likely reason is that you have installed a lot of programs or have accumulated many files. This can often 'scramble' your hard-drive making it slow to react, peform operations (if at all) and greatly reduce it's performance. So I suggest you have a major clean-up of unecessary files and programs and then use the DEFRAGMENT tool located in your main disk properties.
HINT: Be weary of what you install. To be extremely careful, only install programs which supply an un-install fuction.