Ubuntu all the way! Free automatic updates for every application on the system, no viruses (though Macs also have this benefit), easy install, and very customisable. No stupid EULAs either - not for most programs anyway. Only downside is that it takes a little tinkering to get it to play nice with your hardware sometimes, but come Hardy Heron (8.04, will be released in April), it should be ready for most users.

(TBH, I don't think my mum could use it, so it's not ready for the primetime.)

(Random side note - it's so much easier typing on an English QWERTY keyboard than a French AZERTY!)

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