My unscientific boot speed comparison:
FreeBSD 5-Stable/KDE 3.4 - 1:39.13
Windows XP SP2 - 1:26.02
Things to be aware of:
The FreeBSd box required me to login to the console and sype startx to get to the desktop. I stopped the timer when the login screen started up, and started it back up when I entered the 'startx' command.
The FreeBSD machines boots of of a 5400RPM ATA100 drive. The WindowsXP machine boots off of a 7200RPM ATA133 drive
The WindowsXP machine has to load up Avast Antivirus and Microsoft Antispyware at startup. The FreeBSD machine doesn't.
In a few days I will be reloading both systems onto the same ATA133 7200RPM drive. I just got a 3Ware Escalade 7000 RAID controller, and am waiting for a dvd burner to arrive, so I can offload all of my data first. I'll do the same test agian when both systems are booting of the same drive.
Even with all of those things above that make the test uneven, I still think if everything were totally square, hardware wise, XP would still be faster to boot. Take off the Virus software and antispyware software, and XP would only increase it's lead. Sure, you can compare XP to a Linux/BSd distro with some featureless Window manager, but that's not very fair.