That's going to be tough to decrypt because the previous User Name account you had, cyphered the file. I'm afraid that's rather impossible... unless you willing to spend money and a lot of time to do that.
MS Office's got a very strong document encryption, but who's going to spend time decoding it? Is it really that important?
Okay... Then try this:
Check the advanced sharing permission on the shared folder or file.
http://windows.microsoft.com/en-us/windows7/Share-files-with-someone
Try these steps and see if it helps:
a. Take ownership of the folder or file
i. Right-click the file or folder, click Properties, and then click the Security tab.
ii. Click Advanced, and then click the Owner tab.
iii. Click Edit, and then do one of the following:
iv. To change the owner to a user or group that is not listed, click Other users and groups and, in Enter the object name to select (examples), type the name of the user or group, and then click OK.
v. To change the owner to a user or group that is listed, in the Change owner to box, click the new owner.
vi. If you want to take ownership of the contents of the folder, select the Replace owner on subcontainers and objects check box.
vii. Click OK, and then click Yes when you receive the following message: You do not have permission to read the contents of directory folder name. Do you want to replace the directory permissions with permissions granting you Full Control?
viii. All permissions will be replaced if you click Yes.
ix. Click OK, and then reapply the permissions and security settings that you want for the folder and its contents.
Below link for reference:http://windows.microsoft.com/en-us/windows-vista/Troubleshoot-access-denied-when-opening-files-or-folders
b. Give full permission to the folder. Steps to follow
i. Right click on the Folder select Properties
ii. Select Security tab
iii. Choose Edit button(if prompted to click continue, please click on continue)
iv. Select Add type in ‘Everyone’ (without quotes)
v. Click on Check names
vi. Click Ok
vii. Under Group or user name select Everyone
viii. Under Permissions for system check Allow full control.
Let me know the result