A lot of companies actually makes you decide what hardware you want in your PC and then takes a small fee for assembling it. Like you choosing the hardware, but someone else build it for you. Buying a finished PC by Alienware, HP, Acer or ASUS is a bad idea. You never truly know what components you get and you can't configure much by yourself.
If you could give me your location and a possible PC/Hardware/Electronics company close to you, I will gladly help you to get a good performing desktop together. P.s. budget would be great too.