Pelo que li e achei a sua melhor aposta é usar o Windows 8, pois ele suporta o que você gostaria de fazer.
Se o Windows 8 estiver fora de questão, tente usar algo como o Virtual Box Portable. Isso deve ajudar no que você está tentando alcançar.
VirtualBox must normally be installed to run. As a virtual machine program, it needs to install Windows kernel drivers and system services. Like most programs, it also saves its settings in system areas. It can’t just be installed to a USB drive and run on any computer you come across.
Portable VirtualBox is a wrapper for VirtualBox that turns it into a portable application you can install on a USB stick or external hard drive. When you launch Portable VirtualBox on a computer, it will automatically install the appropriate drivers and system services — administrator access is required for this — and automatically uninstall them from the computer when you’re done. It also provides a graphical interface for downloading VirtualBox, setting it up in a portable environment, and changing its options.
Portable VirtualBox is designed to run on Windows host PCs, so don’t expect it to run on Linux or Mac host systems.
Guia:
Step 1: First of all, connect your USB to the machine and backup all the data from the USB drive.
Step 2: Download VirtualBox portable, double-click on the executable file and extract the contents to your USB.
install windows 7 on usb
Step 3: Now, go to the VirtualBox portable folder in USB, and execute the Portable-VirtualBox (.exe) file.
Step 4: From here onwards, you need to follow the onscreen procedure to create a virtual disk and install Windows 7 on USB. Follow the install Windows 7 on VirtualBox guide to complete the process.
Step 5: Once you finish the installation process, you can safely close VirtualBox and remove your USB drive.
Step 6: You can now use this Windows 7 USB drive on any Windows machine that you want. To use Windows 7 on other machine, connect the Windows 7 USB, open USB drive, and run portable-VirtualBox file.
Esses dois links devem ajudar no que você está procurando.