Luke_125 :
Im not going to carry on using the old computer and I didn't upgrade the key came with the computer I still have the key
OK then...
Link the OS to your MS account as per the links above.
If this works, then you go through the Activation troubleshooter after the new PC is built.
However...if this was a
preinstalled Win 10, this may not work.
The only way to know is to try.
But read and do the steps in the links above. See what happens.