I too have seen a noticeable slowdown in system boot since installing SP2 (progress
), but only 30 sec or and a slower initial scan after system start by avast about 90 sec.
But I'm in no rush, I did notice several more XP Processes running after SP2, but I parred them down a bit. It could be the additional security aspects of SP2 and the security centre just monitors your other security programs, so I don't think that would be a great overhead.