The default administrator account in Windows is a built-in account with full administrative privileges. This account was created during the installation of the operating system, and it's recommended to be disabled or at least renamed to improve security. On the other hand, giving admin access to a user account involves granting administrative privileges to an existing user account. This can be done by changing the account type from Standard to Administrator in the security settings. The main difference between the two is that the default administrator account has unrestricted access to the system. In contrast, a user account with admin access has limited privileges and can still be restricted by Group Policy settings or other security measures. Additionally, giving admin access to a user account allows for more granular control over who has administrative privileges, which can help improve security by reducing the number of accounts with full access.