You can install Windows XP on a USB pen so that you can troubleshoot problem or take your office with you. From the article:
All it takes is a minor error in the Windows Registry or a virus infection, and your operating system can become unbootable. But with a properly configured USB flash drive on hand, you’ll always have a compatible replacement no further away than your pocket or keychain. In addition, the flash drive can also provide a secure browser and virus scanner, and lets you take your favorite DVD burning and Office software with you wherever you may go
All that’s needed is a bootable USB Flash drive with at least 256 MB of storage capacity and a Windows Setup CD. Using the program Bart PE Builder (Freeware), you can install Windows XP on the flash drive, along with other software as needed (and as available space permits)…
=> Read more at: Tom’s hardware site…
You can do the same thing with Linux and USB pen drive.