Whether you are a new user or advanced one, whether this is your first computer or second (or even third), be clear as to why you want a computer before you even think of buying one.
Are you only interested in browsing the Internet and checking e-mail? Do you plan to use your PC as a programming machine? Do desktop publishing or video editing feature on your agenda? Or is it a gaming rig?
How much are you willing and able to spend on a personal computer?
After you arrive at a figure, keep a margin of a few hundred. It will give you some room for flexibility. If you are short on money when you buy your computer and cannot add all the peripherals you want, be smart. Buy as much as you can afford.
Don't cut corners on the main system unit (monitor, processor, memory, disk space). Ask which features may be added later, as you can afford them.
Remember, you want the computer to last at least four years. You don't want to run out of disk space or memory in the very first year, all because you trimmed down on the memory or got a smaller hard drive.