Are you going to use your old PC still? Was win 10 on it and was it an upgrade from a previous version of Win 7 or 8?
If you don't want to use old PC and did upgrade from 7 or 8, you can follow this next link and attach your licence to an email address -
reactivating win 10 on new hardware - simply do the 1st step of it before moving, and the 2nd step after.
download the
Windows 10 media creation tool and use it to make a win 10 installer on USB << saves buying it
Does the hdd have win 10 on it now? What you are best doing is backing up anything on it you can't afford to lose and fresh install win 10 on new PC to get the best advantage of the new hardware and avoid old drivers messing with the install. So yes, it would delete all your photos and documents.
follow this guide:
http://www.tenforums.com/tutorials/1950-windows-10-clean-install.html
when you reach the screen asking for licence, click "I don't have a key" and win 10 will continue to install and remain unactivated until you do 2nd part of the reactivation process I linked above
On the screen where you choose where to install win 10, if it gives you an error about GPT drives, delete all the partitions on the hdd and press next. If it still gives error, cancel out of the installer and restart PC and start installer again, it will accept next on that screen this time (some PC just need a restart here)