PC or Mac, this is the ultimate question. If you're unsure of which one to get, you can always get a Mac and install parallels to run both platforms. A PC will ultimately be a little more affordable and have multiple options for a decent DAW.
I have personally had both computers and over the years have settled on a Mac. I started with a G4, then moved to a G5, and now I currently use a Mac Pro. If you're going to run a ProTools HD system, you'll need a computer that can handle PCIe cards. You can buy a chassis to house the PCIe cards, but then that is one more cable and device on your desk.
Be sure to get a computer with a fast processor and at least 8 GB of RAM. The i5 & i7 processors are excellent options for audio production. If you're not quite sure what to choose, call a specialist. I work directly with Joseph Secu at Sweetwater.com (800) 222-4700 x1232.
Also be sure to get a good monitor. With all the mainstream TVs having the ability to connect via HDMI or VGA ports you can have a large selection of monitors to choose from. Being able to see what you're working on is very important. You're going to be starring at this computer screen for hours on end, so it would be a good idea to invest in a screen that doesn't strain your eyes.
Tomorrow we'll cover microphones.