if your using a HDD try defragging it, if its badly corrupted it will decrease performace quite significantly in addition to too many programmes trying to auto-start, try removing some non essential programmes from the auto-start list and see if it makes a difference - if it doesnt work and your able to back up your important files theres always the option of formatting and reinstalling windows so you have the lightening fast fresh install
I agree with the posters above though - if you can afford a 30/40GB SSD that will be the single most noticeable improvement you will notice - on my Samsung F3 it took around 45-50 seconds to boot windows, now with the Skill Pheonix it takes 12 seconds!