nothing, it slows your boot time down,
Enabling task scheduler and the prefetcher will DECREASE boot time. XP after three? boots will determine the order for drivers and other startup items to load. The prefetcher stores this information. It also stores info about programs so they will launch faster.
The main reason for a slow boot time is a specific driver hanging at startup. Have you updated any drivers or added new hardware? You can run M$ bootvis tool to watch how things load. This can show you a driver that is stalling, if you find one, update it.
I should add that EnablePrefetching should be set to 3, that does boot time prefetching and program prefetching.