It appears that a very serious bug in Microsoft Live One Care Anti Virus is causing much grief among users. If it detects a virus on an incoming email, the next time it does a system scan it may in fact delete the entire PST file for Outlook (all versions) or the entire folder store where that accounts email is cached in Outlook Express. And we are talking hard delete, not recycle bin. The only way to recover all your email is via a file recovery utility that scans the hard drive for deleted files, and that assumes that you have not overwritten the file with cached browser files while searching for a solution . . .
Live One Care has not yet fixed this problem - and in fact it may not get fixed until they go to the beta for version 2.x.
AppScout has a full summary of the problem as well as a work around posted. Look near the bottom of that page to find the workaround.
To add insult to injury, Microsofts Live One Care recently came in dead last in a third party comparison test conducted by AV Comparatives.org.
(All links open in a new tab or window.)
Edit: It appears that some, but not all, users were able to retrieve their lost PST file from Live One Care's quarantine. Not clear on details on why the difference, your mileage may vary.