sounds like u do need a bit of a clean up, run defrag, in accesories/sys tools/, it will take a long time sometimes, even over night.
then also run disk clean up, (found in same menu as defrag)
now these are windows default cleaners.try these first and see how you go.
you could also try going to add/remove programmes, and have a bit of a clean up your self removing games/apps you no longer use, ull be surprised how much rubish actually does lie about on your pc.(obviously DO NOT delets any important stuff) using my fail safe way IF U AINT SURE WHAT IT IS LEAVE IT).
and finally m8 the reinstall if indeed you think u need it really isnt all that bad if done correctly (i do 100's a day) and ill be gald to help u along in any way possible.
hope that helps