The basic answer is that if your BIOS boot sequence choices include a USB device as an option, then Yes, you can boot from your external drive. (Just select the USB choice as the first boot device) OTOH, if USB does not appear as an option in the boot sequence choices, then No, you can't boot from your pendrive.
If you're ambitious, you may be interested in this article about RUNT Linux
, which apparently allows you to boot from USB even if the BIOS doesn't support it. Note that it also requires a floppy drive, and this probably would not be for newbies. I have not attempted to use this myself, so I can't offer any feedback, but it seems like an interesting project. -- J.W.