both
Alot of the time they use kinda crappy parts to save money... That's why they're often inexpensive compared to a home-built machine.
But they ALSO do install a lot of useless software that's running on your computer... I really CANNOT stand pre-built machines because they've got all this stuff on it that I don't know what it is or how it got there.
I like to know what everything is on my system and whether or not it should be there. I hate cluttered machines.... grrr