I would like to setup a second PC so that the Hoover in my bedroom is not
running all night long. An older system would be quieter and draw much less
power. Currently trying to get my 9 year old laptop to work over a wireless
network, however with no success. (a laptop would using even less
electricity)
One option to consider is a mini-ITX system. I've got one sitting
here that's pretty quiet in operation (you can get rid of cooling
fans entirely with certain boards although not the one here).
Power requirements are pretty low - the entire system is powered by
a 60W 12V DC PSU.
While the boards aren't particularly cheap the systems usually work
out so because the boards are so integrated - CPU, video, network,
audio etc are all already on-board. ITX cases can be expensive
(but needn't be) - you can use a normal Flex-ATX case if you prefer,
although if you go this route and want a fanless power supply I'd
recommend one of the VIA EPIA TCxxxxx boards which have the DC-DC
convertor on-board instead of using one that came with the case
(you'll still need an external power brick though).
To be fair, performance isn't anything to write home about. Office
software and P2P servents will run fine but they aren't up to
playing most games.