Can't address the battery issue since it would be a major hassle to install XP on my HP Vista laptop because it has the AHCI controller. But, on my desktop that runs both XP and Vista, Vista actually wins the boot race by a few seconds.
This is the same machine, with only the HD as the difference and then the HD's are comparable. From power on through sign-on to desktop is 60 seconds for XP. From power on through sign-on to desktop with sidebar 58 seconds for Vista.
On my HP laptop with Vista it's 65 seconds, and while I have never timed it, it is over 3 1/2 hours based on watching the TV while using the laptop. Fans do not run all the time and are off most of the time. Of course battery life is directly dependant on what you are doing. Wireless connection cuts down the time as does using the optical drive.
Others who have both XP and Vista on the same machine report similar results, the effect is that the boot times are a tie. Some have XP win by a few seconds, some have Vista win by a few seconds. Any PC that takes 5-7 minutes to boot has something seriously wrong, and some have posted these times with XP as well. Usually the first question I ask in a situation like that is are they running an Norton or McAfee applications which are noted for doing this to a machine, both XP and Vista. I fault the manufacturers for loading that stuff, and in fact my laptop came with NIS on it. It was the first thing ripped out. Then I ran PC de-crapifier, then cleaned out some more stuff. I tried to order it without Norton's, but couldn't. As I said it survived about 10 minutes after I turned on my machine and loaded the removal tool.
I respect those who prefer XP, after all it is very familiar to many and I still have an XP only desktop and an XP only laptop, but I found that shortly after upgrading a desktop to Vista is was spending more and more of my time with Vista. When I am on my XP machine I find myself looking for some of the utilities that are in Vista and not XP. I now spend about 90% or more of my time on Vista machines.
XP will have a place for at least another decade, but Vista if properly installed is not the ogre some have made it out to be. I think a properly installed Vista could change some minds, and has on some of the members of this forum. Included is one who bought a new machine and it even (gasp) had an Intel processor even though his avatar advertises the competitor.
Thanks to Solar Wings for the special siggy.