download and use both of these
http://lavasoft.com/products/ad_aware_free.php
http://www.safer-networking.org/en/download/
then
http://www.ccleaner.com/download
(bathsheba - about one a month ... or less)
uninstall anything you haven't used in the last 6 months
finally - defrag
amazing how people see "started slowing down" and immediately reach for the ram ... or a catalog
and the norton uninstaller never fails.
all the above are on a timer ... and only start causing problems after 12 months or so
when I installed X .... slowed down = uninstall X
trying to encode video = fit more ram
want to upgrade my windows 95 to vista = new machine.
started slowing down ... popup windows .... low discspace
fix the bl**dy problem - don't just throw more wood on the fire
I tell my youngest students this ... it's not rocket science
there I fell better now ;-)