Well, I was curious, so I did the same tests. Here are the results ranked from slowest to quickest. The times are to a usable desktop (10 seconds after you first see it in the case of XP). All times measured with a Teletex clock on the TV (nothing else with seconds anywhere nearby

). If you like you can add to all of the below the 6 seconds it takes for the BIOS to stop messing around and get to the GRUB menu.
- Puppy Linux - 54s (took off 3 seconds for login questions)
- Windows XP Pro (32 bit) - 46s
- Arch Linux (64 bit) - 36s (including 8s after login prompt for KDE to start)
- Windows 7 Beta (64 bit) - 32s
This actually surprises me, because Windows 7 seems quicker than that. The only time I've been regularly using it is in the morning before work to get to the internet as fast as I can, but I guess it doesn't really make much difference. Oh well.
Of course, Windows 7 is a new installation, and every one knows Windows start up times approximately double with each update.