theres a reason you didnt find anything is because any reputable site tells people to leave vista alone. its not like xp where you can take off processes to speed up the system. vista is a smart os and needs to be in tact to keep its strengths. the best thing you can do to speed up your boot times and everything else is get the right hardware.
For the CarPC, I would stick with XP and perhaps upgrade to Win 7 when it comes out. As much as I love Vista for everyday use, it is way too slow with regards to boot times to be using it in the CarPC.
yeah the difference in boot times between xp and vista matter much more on the hardware then anything else. even if you have 2ghz 2gb+ machine, some motherboards just dont post and boot as fast as others.
also, i think since most people use ITX solutions with ATOM chips they are sort of crippling themselves from the start. i would imagine a MUCH longer boot time in vista over xp with that type of hardware... but if youre running a desktop chip and board you might even find vista can boot faster then xp. I have noticed xp is faster on new installs, but if you have a loaded system thats been running for a few weeks with a lot of apps on it- vista can get you to a usable desktop just a little more efficiently.