Julia Lerman explains why you need one for Windows. If my laptop did this while I was traveling, I'd be dead:
I just went through a nerve-wracking few hours after doing the latest Windows Update. After rebooting, well, attempting to reboot, all I got was a message that said "NTLDR Missing, Press CTRL-ALT-DEL".
After some digging (and being thankful to have the calm Don Kiely here) it seems that because of a large pile of tmp files on my c: drive, it created a problem for the NTLDR file (NT Loader) that was exacerbated (look it up) by the update.