I'll promote the other way just to be different

I built my previous machine myself, ordered all my parts from the cheapest sources (remember to include shipping costs).
Tip: make sure the processor comes with a fan or heat sink and if it is not pre-assembled that it comes with some thermal paste as well.

Some annoying things to watch for, your case and PSU, the PSU wires might not reach where you want to place your harddrives. The PCI-e cards can be very long and mounting your hard drives in the bottom most brackets in a midi tower case will avoid them being in line with the PCI-e card. But some PSUs will not have long enough wires to reach the bottom of the case.

My motherboard also came with really flimsy SATA cables that broke if you looked at them too hard. Fitting those flimsy cables and then putting in a PCI-e card and trying to avoid pushing against those cables in cramped space was more of a pain than it should have been.

My motherboard also didn't have the I/O shield bundled with it, the little metallic plate that goes at the back over the connecters. By the time I noticed I had already installed the motherboard and I wasn't going to disassemble the whole machine again so I could send the motherboard back. I tried to get a I/O shield out of the supplier but eventually had to give up.

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Last week I started looking for a new machine. I compared prices from lots of different companies. I used their websites to pick all the components I want in my machine and compared the prices. I picked a single company which had a competitive price, was close to me and a good reputation.

I ordered all the components from them, about $2000NZ and asked how much it would be to assemble it which turned out to be $40NZ. Now when spending $2000NZ it just doesn't make sense to me to go through the hassle of building the machine yourself when you can get it professionally done by people that do this for a living.

I ordered it about lunch time, asked them to build it for me and I picked up the machine the same afternoon. I went home, turned it on, checked the specs and then installed the O/S. If I had a problem I could just pop back to them the next day.

$40NZ well spent I think.