I have not got an Acer Aspire, but I have a eeePC and boot from usb all of
the time and don't expect the Acer would be any different. You need to have
the usb key in when you power up. You need to press the boot manager key
(F2?). Then you will get a list of all of the bootable devices. If I put a
Kingston Data Traveller in any usb port then it will show as that in the
list. The order will be determined by the BIOS settings, but it will show
in the list as long as the usb boot feature is turned on in the BIOS.

To make a bootable usb key I use Unetbootin which is available for Linux
and Windows.