Acer Aspire One

Hello,

I have an Acer Aspire One that I installed Windows Vista on, thus I decided after a while that I would rather go back to Windows XP. The recovery portion of the Acer doesn't seem to work so I tried using a simple combo of buying a external CD-ROM drive for the Acer and just used a disk of windows XP. The Windows Setup menu comes on and it goes through until it says it can't find a hard drive on the Acer. Any help would be good or alternative methods.