You'll probably have to call Dell.
Be warned, however, that a pre-installed OS is almost always OEM. They generally have a "generic" Key they use at the factory. It's a non-single user license that the software company (MS) sells to Dell basically at a bulk rate.
This allows the manufactures to ship computers usable. The catch of it is, the end user doesn't actually "own" the OS; he is just allowed to use it. This is more or less spelled out in the EULA.
To legally own your own copy and install it yourself, you pretty much have to buy it yoursel. If you call Dell and cause enough fuss, they might tell you what it is, but I doubt it. Anti-piracy paranoia has risen so high that you just can't get away with that sort of thing like you could five or ten years ago.
If the computer did come with a disk, it's probably only meant for repair and restoration purposes or to be used by a licensed Dell tech guy.
It's the disk is an actually Microsoft Windows CD/DVD (which is really rare) there should be a sticker on the face with the Key on it. If it's not, then your best bet would to just try calling Microsoft.