Small Form Factor PCs

[omegahalo-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org (Chris O'Connell)]

I'm looking for a very small form factor computer to install some home
automation software on.  The software is not very resource intensive.  Here
are the key requirements for the system:
1.  Must be able to power back up  without human intervention if power to
the unit is lost.
2.  Should be small and less energy intensive than a regular PC.
3.  I would like it to be less than $500.
4.  Must be capable of running Windows (so either an AMD or INTEL cpu).

Can anyone make any suggestions about what might work well for me?  I was
looking at the Dell Zino, but am unsure if a better option exists.

Thanks,

Chris O.