It ain't no use to sit and wonder why, babe

Query for the computer savvy:
Why is my laptop suddenly running slower than molasses in January? This started about a month ago, maybe a little less. Sometimes it will be its old speedy self, usually after its been shut down for some time. (Simply restarting it makes no difference.) I've tried deleting extraneous files and potentially harmful programs, run a spyware remover, defragged the harddrive . . . Help?