My backup computers are simply my old, obsolete machines that I've kept running. My dual Pentium III is a machine I built almost ten years ago. My Thinkpad A31 laptop dates from 2002. They are pigs, but they work.

Any old machine can work as a backup machine.

As far as other backup methods, a USB plug-in external hard drive doesn't cost that much nowadays, and most come with backup software. I recommend it.

If you have only a small store of data (many people who do only writing, e-mail, and such fall into this category) a cheap plug-in USB flash drive works great. If you're doing a lot of writing, there's no excuse for not at least backing that up. My book writing files and notes, several years worth including ALL of the drafts and the InDesign versions, comes to less than 200 MB.

On Amazon you can get a 2GB flash drive, ten times that storage space, or about ten bucks. Walgreens carries them.