Mac can run Windows and Linux.
Quite versatile little things.
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Alternatively, you can install a Windows emulator (I'm unsure what it's called -- I know Linux's Windows emulator is Wine, but I doubt it runs on Mac) and run the programs you need through it.
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I personally would suggest an EeePC.
I want one of those things so badly.
[x]You really don't get any smaller than that.
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It runs both Windows XP and Linux (some come default with Mandriva, others come with GNU, and you can also install Fedora, OpenSUSE, and like...a lot more. XD), has a seven to 10.2 inch screen depending on which one you get, and has a fairly decent amount of memory.
[x]I don't know about hard drive space though.
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