Mine came with a full copy of the OS. It has Dell written on it, but it does a clean install - at least my 2k pro CD did. AFAIK, the XP discs we use at work are the same way. The only thing is that I'm pretty sure you have to install it on a Dell product. I've never actually tried to install on a non-Dell, but all their other software is that way, so I assume the OS is too.
Edit: Tried my Dell 2k pro CD in my old Gateway, and it wouldn't work - came up with a message saying it only installs on Dell computers. So it's not a real retail copy, but it does do a clean install on Dell's.