This is a hot-button topic, and people are extremely opinionated about it; it's hard to find someone who's unbiased. I'll try my best to be neutral. I own both PCs and a Mac. I have a Thinkpad PC, a Gateway PC, and a Mac Mini w/ Intel Core Duo processor. The PCs run XP Pro, and the Mac runs OS X (Tiger).
The main advantage of PC is the broad selection of software that's available for them. Some very important software is only available for PC. The main one, in my opinion, is Internet Explorer. There used to be a version for Mac, but Microsoft has stopped producing new versions. Firefox for Mac and Safari are two good Mac web browsers, but you will run into problems occasionally. Some of the features of my online bank only work with Internet Explorer, though they claim they will be fully Mac-compatible by the Spring. Other web pages look bad in Safari or Firefox, but this is generally getting better. But almost every page looks right in Internet Explorer, because the people that write them know that's the browser most people use.
Another program I wish they had on Mac was MS Outlook. They have MS Entourage, which is similar, but not the same. Outlook is becoming the standard in offices, and I am used to it, so I wish they would have it for Mac (Word, Excel, and Powerpoint are all available for Mac.) As far as games go, PCs win hands-down. There are good games available for Mac, but WAY more for PC.
In addition, PCs are generally cheaper than Macs, and if one part of a PC desktop breaks you can just buy a new part and replace it yourself. Most Macs, like the mini, are not even easy to open, much less repair. Those are some pretty big advantages to PCs.
On the other hand, the Mac Operating System (OS X) is clearly better than any version of Windows. The Mac OS wins in terms of stability, ease of use, and attractiveness. Stability is the big one for me. I have to turn my PCs off and reboot them at least once a day. Usually this is because of the Automatic Software Updates (a necessary evil unless you want to risk viruses and malware), but sometimes I have to reboot because the computer is running slowly for no determinable reason and all the software is unresponsive.
When I do boot the PCs, even once the desktop comes up, it takes forever to load programs, and when you do, the programs take a long time to respond. It's like having to warm up a crappy car on a cold morning.
I am talking about relatively new PCs! This is extremely frustrating. Any time I am about to use my PC, I realize that it's not going to be as simple as sitting down and using it. Chances are, I'll have to wait for it to download updates, install the updates, and then restart. I literally try to get to my class 10 minutes early just to get my laptop to start.
This Spring, Mac is releasing the newest version of OS X, and it is going to have a feature called Boot Camp. With Boot Camp, you can purchase and install Windows on a partition of your Mac hard drive, and when you absolutely must use Windows, you just select it at start up. This is the benefit of Apple's switch to Intel chips. But you do have to pay for your own copy of Windows XP.
If I were going to buy a new computer, and I had a decent budget, I'd probably wait a month or two when the new Mac OS X comes out (it's called Leopard, I think) and get a Mac, order a copy of Windows XP on Ebay or something, and install that with boot camp. That could get fairly expensive though, so if I only had 600-700 to spend I'd suck it up and buy another PC. I would not get in a situation where I couldn't run Windows one way or another. I just think it's a necessary evil. But I use my Mac for everything I possibly can.
There is a mistake in one of the other answers. Macs work with two-button scroll mice just fine. You just plug in a regular mouse, and the second button works just like it does on a a PC (i.e. "Context-sensitive menus", copy, paste, etc.), as does the scroll wheel.
Macs come with 1-button mice, for whatever reason, but 2-buttons work fine.