I would recommend Mint for those that are Windows oriented. http://www.linuxmint.com/
Ubuntu and RedHat are the two most popular distros. Almost all distros are able to boot and run from a USB. There are even specialty distros of Linux designed to make repair of Windows and Linux systems easier (http://www.supergrubdisk.org/rescatux/) Since I travel a lot and have to do a lot of my own support, I keep this distro handy.
I know I'll get chided for this but I actually like Ubuntu. (Using it right now as I write this post...)