The method I mentioned is legal and Win10 should use the product key that's in BIOS.  Did you do it this way?:  When product key window appears, click on "I don't have a product key"  Then Win10 uses product key in BIOS.  "when I tried inserting it again" - Tried manually entering product key?  Shouldn't have to. Last year, I ended up doing quite a few Win10 reloads, all with the product key that's in BIOS.  But I have a 7010 and not a 780.  So I'm sure experience could vary there.
Hi everyone!!! So, I have a Dell Optiplex 780 MT and due to some problems with the machine, the OS got corrupted. This said I tried to repair the machine and and reset my system completely. After reseting the OS, it loaded the initial configuration screen for Windows 10 (Country, Language and etc), the problem is that I am unable to do anything because it's not recognizing my keyboard and my mouse. Both my mouse and keyboard are wired so I don't know what problem could this be.  
Mouse and Keyboard problems