Back in 2005 I replaced the motherboard and hard drive on an old HP machine. It was so tight working in the case, that I hardly had any room to move around. Also, I looked at my uncle's Gateway before, and it took us nearly 10 minutes to figure out how to open the dang case. I hate those OEMs who make it next to impossible to even get into the case to do anything.
Oh yeah, i can't tell you how many times I've had OEM machines that I basically did nothing but stare at for 10 or 20 minutes, scratching my head, wondering what to do. That's why I said early that opening the case on some computers is a job in and of itself.
Mac's are far better than PC's. In fact, I've owned a few and have NEVER even had to open them for any reason.