Windows 7 upgrade key

Chappers93

Honorable
Jan 17, 2014
7
0
10,510
I have just ordered up my PC components and left out an OS as I thought I had a copy of Windows 7 Ultimate at home. I managed to find it and it turns out that its an upgrade instead of an OS installation disk. Is there anywhere I can download Windows for free and apply the key when its installed? My only concern is that it states that I need an existing license in order to use the upgrade key.