Mac can run Windows and Linux.
Quite versatile little things.
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.
I personally would suggest an EeePC.
I want one of those things so badly.
[x]You really don't get any smaller than that.
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.