Dell Microsoft product key

If you use a Win 10 iso with build 1511, it should automatically recognize either a Win 7 or Win 8.1 product key in BIOS, and install and get activated.

To be sure you can activate--Windows 10 upgrade should not be clean installed at first. Do the upgrade while windows is running or you might not be able to activate it especially if you have windows 7 or upgraded to windows 8. After activation is confirmed (check it) then you can do a clean install. This is important if you don't have the actual product key right in front of you. Read all the info--

http://windows.microsoft.com/en-us/windows-10/activation-in-windows-10 

There is no "download disk" since you have to make your own install media after downloading the iso file using the Create Media page. 

I haven't found a Windows Product Key sticker anywhere on my recently purchased Dell XPS 8900. However, I successfully obtained the Product Key (and a lot of other useful info) using an application called Belarc Advisor. HTH. 

See here for details:

http://dellwindowsreinstallationguide.com/download-windows-10-oem-and-retail-iso/