many people knock the big companies...and come here asking what's up with this and that...when of course they should know
you can usually diy with pretty much the same parts, and sometimes cheaper than the big names, but you might forego a solid case for a slightly flimsy one, or cheaper peripherels, etc.
diy...you usually get a legit full copy of an OS, usually MS (vista baby) - intention is to, of course, buy your copy and not pirate it...
diy...you have to invest the time to put it together and warrant it yourself if you waste a part...
pre-builts...they do it for you for that little premium for service...but sometimes their service can blow some pretty big chunks.
all works out even depending on your outlook
I love DIY but never pass up the chance to buy some big name machines if you spot one on clearance for 1/3 or 1/2 it's retail value.