Microsoft produced a utility called "pst19upg.exe" which was used to upgrade old versions of "pst" (aka Personal Folders) files to the later version. One side effect of this was the ability to remove passwords from the "pst" file.
Therefore if you find that you cannot access a "pst" file because the client has forgotten their password or has left the company, you can use this utility to get access to the data.
Before using the utility, make certain you have at least three times the size of the "pst" file in temporary free the disk space.
Original "pst" file is 150mb
Copy of "pst" file is 150mb
"psx" file (created during process) is 150mb
New "pst" file 150mb
Therefore an additional 450mb of temporary free space required for process to complete
You will need two utilities:
"pst19upg.exe" - You will need to find this from a third party web site as Microsoft have removed it from theirs. Try this site: Software update downloads
or search on Google. "scanpst.exe" - This can be found on all systems with Outlook installed. Search for it using the built in tools if it isn't on your start menu.