I have had great success installing Ubuntu on old windows machines for friends, its fun and i amazed at how fast it runs and also picks up old hardware drivers. If you are using a google apps, and just need online access it is a great alternative to buying another new machine.
Nonprofits looking for cost saving, or kids that just want to tool around on an old desktop should try it out. You shouldn’t be running xp, and vista is a nightmare, but Linux has some viability in that space, especially with the new cloud-centric atmosphere.
If you have tried Linux in the past and gave up I would definitely recommend a second look, the ease of use is much improved. Hoping to see chrome os become a free alternative for older boxes in the near future.
UPDATE – Peppermint OS linux is a great alternative