If not there are easy ways to extract your product key, google is your friend.
After you have it download the live ISO from microsoft and do a clean install using your 8.1 product key, from what I believe this should work but you might want to do a parallel install to be safe
Nah its a custom built, got the OS fom uni when I was there about 4 years ago. Stupidly thought that just replacing the broken mobo would be simple plug and play, was baffled for days before admitting I needed help and someone told me if you replace the mobo you have to clean install Windows too. Add another 3 days trying to find the old windows cd and then figuring out I had no product key.
Its essentially a different beast now, everything is different bar the RAM, although I still have the original HDD in a drawer so I may be able to get the product key out of that…