Be sure that your BIOS can support booting from the USB device. Some can, some cannot. If you don't see your USB device listed as an option as one of the boot choices, then realistically you would not be able to boot from it.
Not to go on a different tangent, but depending on the size of your laptop's hard drive, you could probably set it up as a dual boot Windows/Slack system. There are frequent threads about setting up dual boot systems here at LQ and a quick review would give you a good idea of what's involved. Good luck with it