At least, you should format the storage, reinstall the OS and install other necessary software (such as wallet software) and never connect to internet again
Alright I will uninstall the windows software and re install it now.
And it's recommended to remove networking component if it's possible.
This I may not do yet since I currently don't have Bitcoin hodlings But I will later on when I have some hodlings to use on the hard wallet.
You did not mention the version of Windows that you are using, but since you describe your device as old, I assume that it is Windows 7. Reinstalling the versions of Windows here may not solve the problem, as there may be vulnerabilities that allow access to your device. You can work with any open source operating system or tails that do not require much experience.
It will only work with Bitcoin/Ethereum/Ethereum based tokens, but if you are planning to airgapped more altcoins, I think it is better to buy a hardware wallet.