Most probably…
avast could be the ‘bad guy’ here. Version 4.5 has some changes and the startup was, in fact, changed by it. Lots of users complain about that.
Startup Delayer (which I recommend myself here a lot of times) is only a workaround.
MS must correct the startup problems, specially under SP2. For security, a lot of subrotines were tighted and startup was slower. They should recognize this either.
So, my experience, the slow startup is due to a combination of XP SP2 + antivirus (in this case, specially the version 4.5) + firewall.
Sometimes, the same occurs on shutdown :-\