You've got too much stuff on it and that is undoubtedly slowing down performance. I don't have time to go into it right now as I have to leave for work soon, but the bottom line is that you need to offload some of your stuff (say, photographs or music) elsewhere and then delete it off your hard drive in order to clear some space.
This means burning CDs or DVDs, or using a flash drive (they are small and cheap, you can buy them at any computer store) or a zip drive or the like.
Or simply deleting things you don't want any more (again, older pictures, music, or perhaps emails with large attachments).
This is where backing up is important. Hard drives, really, should not be used for storage. They should be used for the stuff that you want the computer to do. But in terms of storage, you should be using removable media like CDs, etc. so that your processing power can be devoted more to processing than to storage. Your computer is to be a workhorse, not a garage.