Outlook Sent e-mails deleted from Outlook

[Computer Whimp]

I have lost all of my Sent e-mail twice! Is this due to a virus? a bug? Now at least I learned how to do backups!

Computer:
HP Pavillion 6000 (New)
Vista w/ MS Office (Outlook)
McAfee Security Center, Virus Scan, Firewall, Site Monitor, etc. installed and running