I like the previous post but I think I can add some Real World style... Forget the stale debates and the geek stuff.
If you have an office job and plan on taking work home or vise versa, then stick to whatever OS they use at the office. Think of it as free IT support and software training. You may even score the occasional cool hand me down like a monitor or some expensed software. (MS Office costs a lot! Like $300+!)As for family and friends that share file types made from a specific program like, say, a Pinnacle video editor just for sake of a name, find out what they are and which operating system supports it. Some (many) programs are only supported by one or the other. It is more rare nowadays but it can sure burn you up after a $1k+ computer purchase to find out you can't use the $40 program that mom uses. Keep in mind that any computer is just a tool for running software used to perform a task. If someone you know wants to 'hook you up' with their cool video editing software and maybe even teach you how to use it, (It's way more complicated than any commercial would have you believe) then make sure you get the compatible OS and a system with the hardware to run it. Mac people love to talk about Windows emulators (those are programs that allow you to run Windows stuff on a Mac) but they are not free ($100+) and they don't run Windows programs as well as a Windows machine does. It is my opinion that you should buy a pc that best supports the new hobby or goal you have in mind. Either Mac or XP is totally capable of the basics (email, internet, printing cards, playing DVD's, typing letters, etc.) with hardly any difference in performance. I've got both sitting side by side.
If you or anyone in your family is or plans on being a cutting edge gamer that follows games like teens follow cd releases, (like me) then XP is the ONLY answer. That and a $300+ video card upgrade. Only about 10-20% of major game releases are EVER ported to MAC and usually there is at least a year to wait. Not to say that there aren't some really good games for MAC but if you have to have the latest and greatest that the choice is a no-brainer.
If you are a virtual newbie with no specific plan beyond getting online and learning to work your digital camera then Mac may be a perfectly logical choice. It is a beautiful and stable OS. It is, however a niche OS. Once you are a computer owner you will most likely see Sunday adds with all kinds of cool software titles blowing out from all of the major retailers. Guess what? They are RARELY Mac titles. Mac is certainly cool but there is always a trade off. XP on the other hand, will run almost anything currently made that actually sits on a store shelf.
Mac's are the default pc's as far as professional artists, architects, videographers, movie studios and so on. That is a big lure and Apple really plays on it. (who can blame them?) Remember, these people are doing things that you most likely never will and they are using programs that cost THOUSANDS of $$! Neither Mac's or PC's come with much of anything for free so price out the programs you'll want to use too. XP is not quite the video editing OS that OSX is but it's not that bad and there are dozens of fairly cheap XP titles in almost every major retailer. Both OSes come with basic photo and video editors already installed. The Mac video editior is certainly a better 'free' program. The photo editors, in my opinion, are a draw. I'd give a heads up to the Mac for basic photo file organization. Neither is a deal maker or breaker though as there are so many cheap or even free programs for both OSes that are far superior.
A big deal; Viruses. They exist. I don't worry too much because I'm fairly good at avoiding them in the first place and am pretty regimented in keeping my system clean. Unfortunately, I personally think that more than half of all newbies probably have at least one on their XP machines right now. (please, no debates. I'll admit that I pulled this figure out of the sky and can accept being very wrong) Most are not fatal. Some you'll never know you have. Some are really bad. If you do not want to really learn your machine and how to avoid them, then Mac has a real edge there. You'll see many debates as to whether that is because XP has more holes or because Mac is such a small niche that virus writers can't be bothered. The answer isn't really that important to me. The fact is, at least for now, that viruses are pretty much no issue at all for Mac's.
Last thought; Cost. I really think that life with a Mac is a more expensive one when considering the more limited selection of software titles (Yes, it IS still true) and especially hardware upgrades but it is a little more stable and is better integrated. Is it a lot of either? Nope. I've sucessfully crashed both plenty of times. Speed? Who cares? If you do then you probably already know what you want and need. Set your budget and get the most you can for it. The old rule of buy as much storage, speed and memory as you can afford is still a good rule. You will get more bang for the buck with an XP machine and much less of a chance of file incomapatibilty but there is the whole virus thing too.
Good luck...