If you're using OEM, then you have to have the correct disk.
If you try to load a Gateway PC with a Dell OEM disk, it'll tell you that you can't load it.
But... a little bit of tweaking with the setup files and you can...

One thing that I had weird though...
I was doing some reapirs as a favor for a friend who brought me the tower, and the CDs.

The sticker on the side was a gateway OEM XP home key.
The OS loaded was 2k pro (factory loaded, he doesn't own another computer and this was the first time someone worked on his PC).

The disk they gave him was an OEM XP Pro disk.

Now... I have no keys or cds that match.

I was able to load XP Pro with no key at all and never had to activate it.

Weird..