[Hampshire] Older PCs that have USB Boot for FreeNAS

Hi All,
I want to build a NAS appliance based on FreeNAS (freenas.org). I'm
currently using an old Dell Dimension 4100 for this task. I'm using it
in "Live CD" mode with the config stored on floppy. It works OK but
sadly lacks USB boot. FreeNAS is able to be installed and run from
from a USB flash drive which is more desirable.

Does anyone know of obsolete or otherwise older PCs that can be bought
for bargain prices (say <=£25) that definitely have USB boot?

Many thanks,
Imran