some distros are picky about usb stick vs. cd/dvd (openSUSE 11.3 comes to mind); most of them should work off both. might be obvious to you, but make sure you are burning/writing an image of the .iso file, not just copying it to the cd or usb.

some older PC/Laptops have BIOS's that don't have USB boot option, so in that case, do it via CD/DVD. http://www.petri.co.il/how_to_write_iso_files_to_cd.htm

your aspire one should boot from live usb without problems (F12 to change boot order). it might help if you remove the usb flash and reinsert it before rebooting.