If you have 4 GB USB drive, you can easily carry win7 in UBS flash drive and use it from USB, below are the requirements and instructions you must follow.
1. A USB Flash drive 4GB+ or DVD OR
2. Six GB Pen Drive (its better if you have 8GM Pen Drive and its recommended)
Now please follow the below instructions
a. First of all backup all your data in USB flash drive in on CD/DVD
b. Download Virtual Box Portable-VirtualBox_v4.2.4-Starter_v6.4.9-Win_all.exe. http://www.virtualbox.orgc. Goto USB Flash drive and Run VirtualBox file from VirtualBox folder in you USB flash Drive
d. Create Virtual Drive (just follow the instructions)
e. After creating Virtual Drive install Windows 7 on Virtual Box.
f. When you have done the installation on Virtual Box, remove the flash drive.
g. Put that Flash Drive in any System and use it just like a CD or DVD.
