I've pursued very many potential solutions to no avail. As administrator and owner of the file with all permissions, etc. I still cannot access it nor switch off encryption. Import and restore restore of a new certificate fails to re-establish access.
It's Excel in MS Office 2003 under XP Professional.
Once you encrypt a file, you can't just plop a new certificate on it. Was the password forgotten? Encryption is not the same as user rights. The whole point of encryption is that without a valid password or key you can't break into it. If everyone that wanted to get into an encrypted file can post "hey, how do I break so and so encryption" and get an answer in 4 minutes, not very secure encryption there.
How was the file encrypted? You will probably need the backup of the key from when the file was encrypted to decrypt if the user and password used to encrypt the file were lost.
It's EFS encryption - that which is utilised by selecting "Encrypt..." under advanced options in Explorer. It has nothing to do with passwords in that no password is required (once logged in) to access any encrypted file for which you have appropriate permissions. Thus, the encryption is transparent - no password required. Thus, this has nothing to do with passwords.
From within Microsoft.com, I have followed instructions for trouble-shooting encryption-related issues hence the exploration of permissions. (One needs appropriate permissions to amend security-related properties such as encryption.) From the same site, I also followed instructions related to importing a new certificate. All these things are evidently not unusual activities for an administrator - the role with maximum security privileges and the sort of person to re-establish login passwords for persons wh have accidentally locked themselves out.
At the time of encryption and original usage, I was not aware that encryption depended upon independent “keys” that the system could lose, be corrupted, and could be the subject of their own backup. It's not unreasonable to have assumed that the use of built-in encryption would have some similarly built-in, reliable ability to protect itself from being lost.
It seems unlikely - after much effort in seeking a solution in vain - that there is an answer at this point. I ask just in case someone has solved the same problem.
Although I still have no answer, hang-the-9's response is intelligent and of good merit for reference for other people with questions in this area.