I haven't timed it, but Windows takes forever to boot up. When I used to run Linux, I had tweaked my system to not start unused services like sshd, nfs, rpc, and so on. My Linux system always booted very quickly, from a cold boot to login prompt. But on Windows, it takes a very long time. And Windows isn't done once I get the login prompt - launching Firefox takes a while, too. Since I use the web browser for my work email, that's a long wait before I can start work in the morning.
My wife still runs a Linux laptop at home, and when I upgraded her system to Fedora 10, I opted not to remove the extra services from the boot process. This was mostly because I was being lazy, but partially because I wanted to see how quickly a "vanilla" Fedora 10 system would boot. Her laptop is about 2 years older than the Windows laptop I'm running, and hers boots in far less time.
I think I'll have to run a side-by-side test to compare how long it takes my wife's old laptop and my newer Windows laptop to boot, login, run Firefox, and bring up a web page. It should be very interesting. I'll post the results here.