Since the file is accessed by more than 1 person, it is advisable to change the Attributes of this file to READ ONLY. This way you do not need to write any separate code to keep track of Locking / Unlocking.
Every person who will open the file will get read only access to the...
This sound like you simply upgraded from XP to Win7... not a fresh install.
And this is common for the Files and Folders permissons to be reset.
Locate your folder where you wanted to save the file and re-take the Ownership of that folder by Right Click... Properties and then...