It all depends on what u will use it for. If u are thinking of getting it for college or something like that and want to have it with u all the time then get a notebook/laptop. If u are considering playing the latest games on it, you might as well get a desktop. Another point is, do you want to upgrade the comp at a later point. Laptops for the most part can't be upgraded, you can change the HD, add more ram, use PC cards. Desktops are very much upgradable.
Now Win 2k or XP, both have their probs. 2k isn't that good for gaming and it is not as compatible. XP is ok for playing newer games, forget about DOS or Win 3.1 games. Even some newer games won't run on it either. I tried to Install a game that is about 4-5 years old and it says that it can't run NT based OS. XP is more compatible though.