When you get to the part of the installation that asks you to enter a product key, select the 'I don't have a product key' option.
A better alternative to the standard hard drive, if you can live with less storage space would be to install a 500 GB solid state drive (SSD) which provide much better performance over a standard mechanical hard drive.
This one comes with a 3.5" to 2.5" mounting bracket, which you will need to properly mount the 2.5" drive where the 3.5" drive goes.
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