Assuming no spy or malware is causing it...see the sticky at the top of this forum.
Sounds like a bad case of
cruft.
Clearing out files you don't need and defragging the drive will help apparent speed, it will also free up space for the windows swap file which can also help.
Making sure only programmes you are acually using / are likley to use are loaded, especially started on boot up, will also help a great deal.
Ok - not adding much, but the opportunity to use the word cruft could not be passed
