If you find it hard to find a computer that suits you, or you need to keep updating it to run the latest software, you could consider custom building your own machine.
With a custom built machine you have the luxury of being able to choose exactly the components you want. This means you'll never either under power your machine again, simply because a manufacturer thought a certain part would do the job for most users, or over pimping it just because the manufacturer had a deal with a certain parts maker.
We could argue whether the motherboard, central processing unit, RAM or computer case comes first but the reality of the situation is that you need to decide on all four together. It's pointless to buy a case only to find it won't fit the motherboard you have decided upon, or the motherboard only to discover the processor you're after doesn't fit in it, or the sort of RAM needed to run the most vital software on your computer isn't supported by the board.
You might want to start with the prime software you want to run on it and work back from there. And if you want to future proof your machine look at those requirements and notch it up one or two rungs to future proof it. That way when the newer version of the software arrives in a year, or two, and it's more system hungry it should more than suffice. If it doesn't it's pretty easy to upgrade the RAM or the processor and much more cost effective than buying a new machine.
Consider what other things you'd like to do with your machine. The beauty of the modern motherboard is that it comes with all manner of expansion slots that lets you add a television tuner, graphics card, or wireless functionality with the click of a component. If you have lots of functionality in mind then a motherboard with the most slots is needed.
No computer is possible to build without a hard disk drive. Not only is there multiple choices in terms of storage size, but there's also plenty of choice in terms of technology. There's the old fashioned hard disk drive, still the mainstay of many machines. There's also the solid state drive which, as it's name suggests, has no moving parts so is quicker to access data from. There's also hybrid drives which mix functionality.
To get most bang for your buck it's worth installing the operating system, Microsoft Windows Windows 8.1, on a solid state drive, and keeping documents on a more traditional hard disk drive. You could even put two of those in - one to back up to the other in case of a drive failure.
Do you need an optical drive? These days it's possible to download all your software from the internet. But you might want to go for a DVD or Bluray player if you watch a lot of movies on your computer. Be careful here as a Bluray player is more resource hungry than a DVD player because it needs more computing power to deliver high definition video to the screen.
Here, too, is another consideration. A high definition screen will serve you well, and not require upgrading as often as some of the parts inside your machine.
When you've got all the parts it can be immense fun putting them all together, installing Windows and booting the machine up for the first time. But if you want your machine to come with a warranty it's better to tell us what you want and we'll get it built for you.