For many distro's you simply make a usb just like it was a real hard drive. If you are careful you can use each installer to the usb just as if it were a real drive. Be sure to know how to save windows 7 first. I disconnect the internal drive or suggest a VM to use. Don't try this with internal disk attached is my suggestion.
There are things like yumi on
www.pendrivelinux.com and many other web sites as noted above. Some guys like to make real complex builds but it does require too much work for my tastes. Gets complex when trying to modify them or such. Some speciality distro's like puppy are not typical distros.
How much does a 4G or 8G flash cost anyway? You can buy a pack of them for almost nothing. I mush have 20 of them laying around from an old 128M with a distro on it to a few 16G with modern BTRFS filesystem on it. Best part of many sticks is that if it breaks or gets lost no big deal.
