I recently made a low-cost backup computer for a customer who couldn’t afford to have her PC fail and be off the air for a few day.
What I did was use an Acer One Netbook (windows XP), a new LCD monitor, a keyboard, mouse, and a WiFi router.
I then setup the Netbook, so that it would use the external monitor, installed logmein, installed syncback, changed the power settings, so that closing the netbook lid would not send it into standby.
Now the netbook can sit unobtrusively behind the monitor, is networked with the main PC, and it runs a daily backup of: My Documents, Desktop, and the Outlook Express email database.
Now if any component of the main PC breaks, it can be replaced with the backup PC.
And I can occasionally connect to the netbook from “home”, via logmein, to make sure the backups are running correctly.