Question : outlook.pst file disappears, cannot be restored via backup

Twice on the same computer, after a normal shut down, the user, upon going back into outlook found that they couldn't open outlook, and sure enough, the outlook.pst file was missing from the normal place
(c/documents and settings/username/local settings/application data/microsoft/outlook)
Both times, I took the backup copy off of our NAS device.  Both times the outlook.pst file was corrupt and scanpst could not bring it back (the scan failed and died in the middle of scanning).  The backup software we are using is Memeo Autobackup which backups outlook every day.  
This computer was just set up from scratch (new os - XP Pro) days before the first time it happened.
I can't see what could have caused this and I am blaming Memeo Autobackup as it is the only thing running which touches this file. However, I have this running on at least 30 other computers, and have run it for over a year on many, and have never seen anything like this with Memeo.
Does anyone have any other ideas as to what would cause a pst file to be deleted off of a computer and to be corrupted in such as way as to be unrestorable?  the pst file is relatively small, about 100MB so it is not a case of a too-large pst file.

Answer : outlook.pst file disappears, cannot be restored via backup

Memeo seemed to be the culprit.  After removing Memeo Autobackup from this computer, outlook.pst stopped being deleted.  In the meantime, it occured on another computer I had just put Memeo on.  So if you use Memeo autobackup, beware that it can sometimes, strangely enough, delete outlook.pst if you are using it to backup outlook.pst!  Oh, the irony!