Well, if Windows XP is like its cousin, Windows 2000, it should be remarkably stable. Windows 2000 does have issues with multimedia performance and, to a degree, backward compatibility, but Windows XP is supposed to combine the stability of Windows 2000 with the speed and multimedia friendly aspects of Windows ME. So it should be the best incarnation of Windows yet. From all indications, it has lived up to its promises for once.
As for "big," it requires 1.5 GB of space. Considering the average retail hard drive is now 40 GB, with 80 GB hard drives becoming more reasonably priced (and 120 GB hard drives just starting to come out), space should not be much of an issue. If you have an older computer (as in Pentium II era), you might have the lower capacity hard drives (< 5 GB) and you may have to upgrade.
The last issue is that if you are a Windows 9x/ME user, you will probably have to locate XP compatible drivers for some of your hardware devices, since the Windows 9x/ME drivers will not work with it. Luckily, many of these drivers are likely included on the CD and the rest are downloadable from the specific hardware developer's website.
Basically, the only thing I gripe about is the "Big Brother" nature of its activation crap, not to mention the very unrevolutionary "new additions." It is pretty much Windows 2000 SP3 with pretty colors, in my opinion. However, if you don't like the colors, you can change it back to "retro" Windows 9x/ME/2000 colors, and if you are very adamantly against the activation, there are ways around it of questionable legality...but you'll have to figure that out yourself.
Melon
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"He had lived through an age when men and women with energy and ruthlessness but without much ability or persistence excelled. And even though most of them had gone under, their ignorance had confused Roy, making him wonder whether the things he had striven to learn, and thought of as 'culture,' were irrelevant. Everything was supposed to be the same: commercials, Beethoven's late quartets, pop records, shopfronts, Freud, multi-coloured hair. Greatness, comparison, value, depth: gone, gone, gone. Anything could give some pleasure; he saw that. But not everything provided the sustenance of a deeper understanding." - Hanif Kureishi, Love in a Blue Time