Your BIOS must support booting from USB devices. On system startup you need to get into your BIOS (usually F2 - check manufacturer's specification), and find where you can specify the boot order. Select a low-numbered boot order item, and set it to USB-HDD (if availble, otherwise another USB option). This may require several attempts until the right device option is found.
Since most USB drives are ATAPI-compliant, your BIOS should detect it as if it were an external USB hard drive.
Just make sure you specify a boot order BEFORE your internal hard drive (usually HDD, or IDE0), otherwise your system will always boot to the hard drive.
Also, you need to format the USB drive as a bootable drive. If you can get the system to boot from a DOS, Windows or Linux floppy disk, you can try installing DOS, Windows 95/98, or Linux (Win2000, and WinXP require more than 256Mb for system files). Your system will likely assign the USB drive as the C:\ drive (MAKE SURE THIS IS THE CASE BEFORE FORMATTING!!!).
Keep in mind that most USB drives are slower than hard drives, so performance may not be up to par. The newer "high-speed" drives are faster, but not comparable to a real hard drive.
If no options work, your Micro Valut may not be ATAPI-compliant, or it's USB-HDD firmware may not be compatible with your system. I had this happen with a SanDisk Cruzer mini, but a PNY Attache worked.
If I may make one suggestion as well...make a bootable CD with your images instead. CD's are much more rugged, and cannot be accidentally erased. Duplicates are EXTREMELY cheaper (about 5 cents vs. $20) than the USB drive solution. Many more BIOS' support booting from CD (for about 10 years now), so compatibility is usually not an issue. All of the commercial CD-burning packages (including most of the LITE versions) support making bootable CD's. The only trick is to make a bootable floppy that it can use to create the boot image on the CD.