Most BIOSes nowadays have a feature whereby pressing a specific F key brings up a one-time boot device selection menu, handy for operating system (re)installs. The BIOS on my new Vaio didn't display any information regarding keys (not even how to get into the BIOS). After blindly trying escape and del, F2 got me into the BIOS. There was no one-time boot option in the BIOS so I did a google search. A random post in a random thread said to try F11, which cycles the boot device through the available media.
I was trying to boot from USB: the first time I pressed F11 during boot the laptop tried booting from the empty CD drive. The second time (I can't remember if the laptop rebooted itself, or if I had to) I pressed F11 it successfully booted from the USB.
- Aforementioned random post: Sony Vaio select boot device