Windows 7: Upgraded Serial Number

Hello. I upgraded into windows 7 from one of my computers at home. I need to know which serial number I upgraded from because I [EDIT: NEED TO] reformat and now I need to go through the process of installing the old operating system, then doing a clean install of Windows 7 while the old operating system is installed and registered. So is there any registry trick or stored file somewhere that I can retrieve which of my old windows numbers I upgraded from?

Rant: This process is somewhat frustrating

What kind of Windows 7 install disk do you have? Is it retail or upgrade?

If by upgrade you mean, is it upgrade only, then yeah, it is upgrade only which is why I have to install my old operating system before I re-upgrade. If by retail you mean did I get it from walmart, then its retail.

Oops I made a mistake in my original post. I still have my windows 7 install but it is not yet reformatted, so from it I would like to retrieve the key I used to upgrade it. I know which windows 7 key I used, but not the one from before that.

No you dont have to. See here how to.

Clean Install with a Upgrade Windows 7 Version

But this is legal only if you own a previous qualifying OS (need not be installed). So if you have one, you're fine.

Thanks for your help, but that isn't entirely the problem. I can install windows 7.

When I try to activate I get this error:
0xC004F061

Ignore what I wrote above. That website tells me how to fix that problem too. Thank you very much I appreciate your time and effort.

worth to have a look at this one

Windows 7 activation error: 0xC004F061