1. What do you want to do with it?
2. What’s your budget?
3. Are you technically minded?
   A “Yes” might lead you down the path of Linux and a “No-brand” or “White box” system.
   A “Yes” means you are able to do your own support.
   A “No”, buy a Dell*.  As they’ll be providing your hardware support.
4. New or re-furnished?
   If you want to save a dollar, a re-furnished PC from Dell or IBM might be an option for you.
   If you are a member of a trade union, your Union might have a “Union Shopper” type deal.

* Heck, I got 5 years out of my last Dell.  And my current work computer is a Dell, which I’m quite happy with.  I’ve listed out the various factory outlet Dell sites here.