1. Being logged in as admin does not guarantee you have permissions to alter files. If admin permissions have been revoked on a file, leaving only system permissions, then you won't be able to do anything to update the file.
Simplest test is to grab a copy of the updated file and try and copy it over the existing file while logged in as admin. If you cannot do it, then you at least know where to look for the issue. 

2. Being on a standalone machine as opposed to a networked machine may not make any difference to whether the file is locked, if it happens to be started as a service or by some other startup process during boot.

3. Perform an MD5 checksum on your original version of this file, and compare it with the checksum of the file on the test system you are trying to update.  Out of interest, was the original file an exe that was installed over the internet?

4. Check your antivirus and antispyware solution to make sure it is not blocking the update to your presumed executable - as this can be mistaken as virus activity.

5. If posting again, let us know more details of the type of file you are updating, and the target operating system. If it is Vista, let us know if the same issue persists when you turn off UAC.