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... put Ubuntu or Knoppix or whatever flavor of Linux floats your boat on it. Free!
Sure it can be an adventure trying to get things like Wine (the Windows emulator) or various pieces of hardware running on it. If that frightens you, go only with vanilla hardware and the simplest distribution you can find.
But it honestly won't take long before you realize that 90% of the things you use a computer for, like reading nonsensical tech-infused rantings from a writer who convinced Salon's higher-ups that he was genuinely savvy about technology, can be done just as easily from a Linux Firefox window as a Windows Firefox window.
I never sell computers. I had to admit to the insurance company that some uneducated thief stole my 10-year old Pentium/150 Tecra, a machine that's worth $30 on a good day. But for me, that was the machine that I could bring out and talk to my car's electronics, and I didn't care how much danger it was in.