Windows File Permission common practices
Solution 1:
If the administrators don't have an ACL giving them permission, they won't be able to access the file. But the administrators can take ownership of files, thus being able to set its permissions.
If you really want to prevent unauthorized access, you should use encryption. Note Windows EFS with keys issued by an internal Microsoft CA can also possibly be accessed by the CA administrators etc., so you might be looking for Truecrypt or Bitlocker (not sure how that one works in a "corporate environment") or GPG.