If you know someone who goes to Temple University, you can get a nice backup system for $50 ($25 for monitor, $25 for CPU). They usually come with a clean install of Windows XP SP2.

Here's the site: http://crc.temple.edu/ (Temple's Computer Recycling Center)

For example, two systems available on there right now are a Gateway E-4600 (1.3Ghz with 512M of mem) and a Apple PPC G4 (733Mhz with 512M of mem).