really slow start up

from xp progress bar to active desktop takes forever and a bit.
once desktop appears it takes 20 seconds or more for icons and tray bar to appear, then ages to load a few progs to tray. it also takes far to long to load any progs i select.
i have no viruses or spyware, unless via re-root kits which i know nothing about.
defraged, reg cleaned and rubbish cleared.
any ideas please
slower