If you really are logged on as Administrator, you should have access to all files as long as you have satisfied certain criteria. And the fact that there was a previous administrator should not mean that you are barred from accessing those files. But you may have to fiddle with the Permissions. Passwords are another matter but the following should get you some way along the road.
First, open Windows Explorer and highlight "My Computer"with a single left click. Left-click "Tools" on the Menu Bar; left-click "Folder Options" and then left-click the "View" tab. Scroll to the bottom and uncheck "Use Simple file Sharing (Recommended)" if it is checked.
If you were able to do this then the way ahead is easy. From now on when you look at the properties of any file or folder, there will be an extra tab along the top. This will be the Security tab. Click on that and put ticks in any and all boxes for the folders and files that you can't access at the moment.