I resurrect my old PCs with Puppy Linux. This is beautiful little version of Linux that is so compact it runs COMPLETELY in 256MB of RAM, including the OS, browser, email, paint, spreadsheet, audio program, photo program, etc, etc. It makes your oldest computer the fastest. Download and load it on a CD or Flash drive and try it out. It is just amazing and so easy to use. For the 99% that just want to do Web browsing and email, it is PERFECT.
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Meanwhile, if you have a higher powered computer and want to be free of Microsoft bloatware, the latest Ubuntu seems to be really great, stable and complete.
Of course, both versions are Linux, and can run any of the hundreds of Linux programs that are out there, including the ones that are compatible with the MS versions (Office, etc.)
In addition, there is a GREAT version of Ubuntu that I hear is very similar to having ProTools.