Hello, thank you for reaching out to Microsoft Q&A.
Since it is a clean installation, you can set up Windows using another Microsoft account. If you don't have one, you can create a new one using the link here - https://signup.live.com/
Regarding the Windows account that your late grandad created, you do not have any recovery methods, such as a phone number or email address, to verify your identity? Microsoft will verify your identity when you make security changes to your account, like resetting your account password.
If you do not have access to the recovery methods, you will need to use the account recovery form. Once Microsoft verifies your information, you should be able to update your security details and reset your password. Please continue submitting the account recovery form until Microsoft is able to verify your account.
Remember to use a familiar device from which you've frequently accessed your Microsoft account, as well as a familiar location. Microsoft will review your responses and respond within 24 hours. If you are verified, instructions on how to log back into your account will be sent to you, and if you are not verified, you will need to submit the Account Recovery form again. You can do this up to 2 times every 24 hours.