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.

I did that and I have the following 6 boot devices.
1)USB CDROM (which I do not have installed)
2)USB FDD which I presume is the floppy -- again I have no floppy
3)USB HDD which I presume is a Hard Drive -- again no hd attached.
4)IDE0 which is my HD
5)IDE1 with nothing attached
6) Network Boot : Atheros Boot agent.

I have a wireless logitech mouse attached to one of the usb
I have my ham radio connected to the 2nd usb
and I have a 2G stick on the 3rd.

I formated the 2G to fat (hopefully fat32)
I have unebootin on another drive which I tried to load ubuntu on but it
keeps telling me that there is no usb drive installed or it needs to be
formated fat32. I already did a format on it (but it didn't specify
16,32 or whatever.

USB HDD also means any external bootable USB device such as a flash
drive, so try that

there is a ubuntu community doc page specifically for the aspire
ones.

https://help.ubuntu.com/community/AspireOne

I'm on mobile, so didn't read much to see if there is quality information
there or not, but thought I would share incase turns out to be useful for
you.

Yes, I have used several usb sticks to boot on my AOA150 currently with Ubuntu
11.10. Strangely the software 'Startup Disk Creator' is fussy about which .iso
it will accept, it does ubuntu isos but not all others, so if you have trouble
try Unetbootin from the Software Centre. Until you get it working I suggest you
remove the other USB devices.
Occasion usb sticks can be difficult so I sometimes set up and format partitions
first using GParted and mark them bootable.

When using SD cards to boot,I find it easier the put them in an usb adapter.
Suggest removing the redundant devices fron the bios menu and leave usb drive at
the top, otherwise us F12 immediately after the bios to get the bootup menu.
You'll find Puppy great, woof!