System Restore is a godsend from a Tech Support point of view, fixed too many corrupt profiles that a normal editing the profilelist entry via regedit doesnt fix.
My favourite tweak is turning off windows search (indexing), really cba that starting up and slowing the machine down as Im trying to do something important.
Oh and setting Windows Update to manual instead of automatic can save reboot/cold startup pain waiting for never ending automatic updates to install when you least want them too.