I'd go with Linux, specifically Puppy Linux. It's a tremendous little distribution- 130 MB or so- and even once I fill it up with all kinds of crap- like KDE, several office suites, a/v editors and players, several web browsers, CAD, etc.- I still only have a system totaling maybe 4.25 GB, tops. (From memory, when I used to setup Windows XP with a similar set of applications, it took up something like 20 to 30 GB, depending on how I installed the OS updates. The much-vaunted Ubuntu (8.04) took up about 14 GB- still comparatively steep.)
ReactOS looks interesting, though, assuming they ever get beyond alpha releases sometime before we all die of old age.