Hi!

In 2016 I bought a Dell Vostro Desktop that came with Windows 7 installed but funnily enough it came with a CD with Windows 8.1 that says OEM activation key required which is at the back of the CD pack. After all these years I decided to have a go at installing the Windows 8.1 CD, however when I'm prompted to enter the key, I encounter an instruction to insert a retail 25 character key, as opposed to the 20 character OEM key. Doesn't even type in numbers so I don't even get the chance to try the OEM activation key. 

Is there anyone who can help? Any suggestions?