About building your own PC, it could be really smooth like Harley described.
Or it could cause you so much grief.
Both have happened to me, and I am somewhat knowledgeable in this, having built and updated numerous PCs for my own personal as well as business uses.
Example: A motherboard had two RAM slots. Got two sticks of RAM to populate both slots. Windows crashed randomly, after anywhere from 2 hrs to 1 day of running. Application bugs? XP updates missing? Graphics, sound, network drivers need download updates? Did all that. Swapped out graphic board to try another one. Software updates all over again. Same thing.
By the way, when it crashed, it was usually with one particular application program.
Took several days to finally suspect RAM problem. Slowed down RAM access speeds. No effects. Ran various exhaustive RAM diagnostic tests, which found nothing.
From a hint found on the Web, pulled one of the RAM sticks. Seemed to work (meaning had to wait a day or two to be sure). Out of curiosity, try each of the RAM sticks in each of the two slots (4 combinations). OK in each case.
So, just can't have both slots occupied! Design errors? Board problems? RAM problems? At this point, got too tired to return the board, and just ran with 1/2 the RAM I intended. Eventually, with RAM price dropping, bought a RAM of twice the density to plug in one slot.
I have more patience and curiosity than most to work this out (Remember that I do my own car repairs). I also have plenty of parts on hand to troubleshoot by trial-and-error swapping.
Even being an EE, I would need expensive logic analyzers and LOTS of time to track down the deficient signal timing of this particular motherboard. This is something to be done by this board's designers, not by the user.
Your mileage varies GREATLY!