I know this is easy to do these days with Linux, is there any way to do it with Windows 7? I don't mean install it from a thumb drive, but run it from one.
Technically, no. Microsoft prevents Windows 7 from being installed to an external drive.
I found a forum that details using VMware to install Windows Vista (process is supposed to be the same for Windows 7) to an external harddrive, and then doing a little registry editing and convincing the machine to boot from the external drive to get it to work. Apparently it requires a harddrive in the computer while booting (some type of Windows verification) and has a few USB problems.
The Into Windows blog has a sort of "work around", using VirtualBox Portable to create a virtual machine on the drive. Having VirtualBox Portable on the drive allows you to boot up the virtual machine from other computers without needing to have VirtualBox installed on the machine1. This would only work for a Windows-based host machine anyways.
Realistically, I think your best bet would be to set up a virtual machine if you want to carry a Windows installation around with you. Windows really isn't designed to be run from a USB/external drive.
1 I'm not sure if this would be possible without running as an administrator on the host machine - I know that VirtualBox requires access to the hardware, so this may not work on a limited user account.