MS-DOS 2.21! Rock solid, and FAST!!!
Used to do a multi-boot of Win7, Vista, XP, Kubuntu, Ubuntustudio, Fedora, and FreeDOS. Was fun, but the administration of it all took too much time. Now I just run Win7 and XP in a vm in Ubuntu.
I know, I know, I'm so lost when it comes to the mac.