I can't do more than quote from the manual:
[h=4]Changing Boot Sequence for the Current Boot[/h]You can use this feature, for example, to restart your computer to a USB device, such as a floppy drive, memory key, or CD-RW drive.
If you are booting to a USB device, connect the USB device to a USB connector.
Turn on (or restart) your computer.
When F2 = Setup, F12 = Boot Menu appears in the upper-right corner of the screen, press <F12>.
If you wait too long and the operating system logo appears, continue to wait until you see the Microsoft Windows desktop, then shut down your computer (see Turning Off Your Computer) and try again.
The Boot Menu appears, listing all available boot devices.
Use the arrow keys to select the appropriate device (for the current boot only).