Basically, for a cheap box with all new parts and a legal copy of Windows (at least in the US) you can't beat a pre-built system on price by building your own, or even come all that close. For a midrange system it's about even.
I see. Well, that's definitely not the case here. While there are plenty of companies around that sell pre-built systems (including Dell, HP, Fujitsu, and so on), the only reasonable use of those I can think of is for businesses, so the computers are all roughly the same, and because of long-running service contracts (which I would never consider for a home computer).
Other than that custom always wins... price, performance, quality, upgradeability, in every regard really.
For a mere €20 I get all the parts I want put together professionally including testing, OS pre-installation, and so on, and with a 2yr warranty on the full system on top (which is more than enough to cover the occasional defective hardware). I can even bring some of my own parts from somewhere else (as I did last time with the case, since they didn't have the one I wanted, anymore

).
And building a high end box is crazy unless you need it for work, in which case you want someone else to do it so there's a support contract in place, or you're a hardcore PC gamer (in which case, yes, you should build your own).
Don't even need to be a hardcore gamer. I wouldn't consider myself as such, yet I have a fairly high end machine (or had, one and a half years ago; it still runs all the new games, though

).
But maybe the situation is different here, quite possible.
You can buy System Builder / OEM Windows seperately here. It's illegal to restrict them to a specific machine.
Bye
Thanee