I know this sounds crazy, but your hardware has a lot more to do with this than the OS. I've seen both xp and 7 boot blazing fast but their hardware was optimized to boot quickly with that particular OS. I'm not suggesting that you upgrade your hardware, but it's one of those deals where we could tell you ______ boots the fastest, you install it on your particular machine, and it's slow.
Xp is your best bet generically speaking, there is just less stuff to load, but regular xp, not tiny.... that stuff is garbage.
I agree entirely. If you have the dosh, upgrade. If not install a fresh copy of XP.
If you restrict your hard drive to Windows XP, Ems and Roms it should be pretty nippy. Beauty of Mame and this type of retro gaming in general is even modest computers are more than enough to power all the old games.
Of course there are some Motherboards which boot pretty much instantly but then your talking a lot of money.
You could do yourself a favour by having at least 1 gig of ram. If you can 2. IF your not going online then don't go adding any virus protection because that is an absolute killer to start up and speed in general. Also a thought to remember is you can REPLACE Exlorer (windows browsing-desktop/etc) with your chosen Front End thus getting instant access to your gaming system once it has booted. And when you crash out of any of your emulators it goes back to it.
I have XP, I have 1.5 Gig ram and a 80gb hard drive. Its smooth as works for everything I need (Games 1978-2001) and with the odd bios tweak it boots in 25 seconds. Probably get a few seconds off.
You must remember every time you add software you get registery keys being added. A lot of programs use the prefetch option to load part of themselves before the operating system starts- major programs do this, Office does it, Acrobat others. Using vital memory and resources.
N.B: You can hide all the windows stuff very easily on XP so it loads with your image instaid of the XP loading screen and all that.