Overwriting
You should get a machine with nothing on it but its bios and install an OS from scratch. This is the ONLY way to be sure of getting a good, clean install.
This becomes a real pain if your computer has certain bells and whistles requiring special software. My laptop only works right if you rebuild it using its original "restoration" disks, and of course, that usually installs some form of Windows along the line.
Any reputable turnkey dealer should allow you a choice of operating systems. Within reason--nobody's going to sell you Win95.