I remember a while back dealing with pst's reaching a critical size and causing problems. Your client should be running a backup or redundancy process to protect from lost data. It is possible for the outlook to write to the pst and crash and kill the pst file in which it can be written as a 1k file. Just creating an empty file creates a 1k file. I would suggest implementing a backup procedure for your client either daily, weekly, or monthly backups depending on how critical the data is to protect from deletion. If your lucky his system might be able to get the pst back by restoring the system back to an earlier recovery date...BUT be sure to save all recent data to a thumb drive and remove it from system. Bring system back to date when his pst is functional, then roll recent data back to the system. Backup the PST's to thumb drive in case ccleaner or driver removal has a strange behavior of affecting the PST, then safely remove thumb drive. You may have to perform your ccleaner and that printer driver removal again as for it will restore what you worked on to remove. Then verify his pst is there and move on.