I say build your own computer and go with Windows XP...if...if you have the inclination to learn about computer components and how to build a computer. I suggest starting off at the Anandtech.com discussion forums.
anandtech is terrible! well, the forums are okay, they're just forums after all. but for better reviews and such, i recommend hardocp.com
as for the OP...
the biggest issue to answering your question is: what do you want to do with this comptuer?
Dell and Apple are both over priced for what you get; Dell isn't quite as bad, but they tend to use a few inferior parts (power supplies, slow RAM) to keep costs down. i personally dislike Mac OS, and i feel their hardware choices are, well, strange.
+1 for building your own rig. it really, really isn't that hard. for the most part, it's just a matter of plugging things in to match sockets (it can be a little more tricky than that at times, but there's always help to be had on this here interweb).
i've managed to teach myself quite a bit about it in less than 6 months, and i recently went into part-time business with a friend building custom computers locally (San Diego, CA).