1. Yes, you must you the same license
2. Reinstall
Acer normally delivers the desktops with "Acer eRecovery.." installed. The quickest way to reinstall is selecting the option "Restore to factory default" in that program. Your license will also be restored.
There are more possibilities. There may be a recovery partition on your disk. See your manual on how to enable it in BIOS, and when enabled, press ALT and F10 when the Acer logo appears at start up. Again, this should be explained in your manual. Your license will also be restored
If that all fails you will need the installation CD/DVD/download of Windows, Office and whatever you had installed. You will find drivers and utility programs on the support pages of Acer. There are...