Since when does reformatting give a speed boost? Stock windows is slow and needs a lot of work done shutting useless things down to get any decent speed out of it.
Just go through Add/Remove Programs to get rid of stuff you don't use, run a registry cleaner, and defrag.
Um Windows XP Pro runs very fast when first installed. In fact my old system would load it from post in about 10 seconds. There is normally nothing useless when its first installed even with SP2. Now yes you can go through and turn off a lot of services to boost its speed but on that system it will be very fast.
As for Vista, it does take a bit more to load but it runs nice and fast after all the updates, especially SP1.