yeah SP2 slows down pc performance alot, be sure and defragment your drive it helps alot, and kill excess processes you dont really need starting up at bootup. only things that need to load up at start up to work properly are defrag programs and antivirus programs, everything else should be fine. just google the process name and you will figure out what it does and it you really need it
what kind of net connection do you have dialup or broadband? are you behind a router? if so disable the junky windows firewall its not gonna stop any hacker anyways. also kill the security center process its not needed.