One of the 'features' of vista is that it appears to boot up quickly. This is somewhat a false impression. It loads enough to get it started straight-away and continues to load additional items afterwards. Some programs have much more overhead than what is needed for startup.

Does the overload issue seem more prevalent when connected to the net? It could also be that any number of things are trying to update themselves. Have you noticed a lot of hard drive activity during this period

As sbm12 suggested, watch your task manager.