It's always worth noting to newbies that airgapping is most ideal in Linux than in Windows, for obvious reasons, including avoiding a plethora of malware and other assorted threats.

Even if your computer is airgapped, and has never touched the internet since you installed the OS, you still can get a virus via USB, CD, etc, so dealing only with open source software will always reduce these possible exploits by a big %.

Look into Ubuntu or Lubuntu if your computer is very old. You can always use Xubuntu as a good balance between performance and aesthetics.