On Wednesday, Microsoft owned up to the bug. When OneCare's anti-virus scanner detects malicious code in an e-mailed file attachment, said the company, it may quarantine Outlook's or Outlook Express' data file, making it appear that all the messages have been erased.

"We are working to address an issue where the anti-malware engine for OneCare is erroneously quarantining Outlook .pst files or Outlook Express .dbx files, when the .pst file or .dbx file contains an infected attachment," a Microsoft spokesperson said. The fix will be included in the next scanning engine update, which is due to roll out March 13.

In the meantime, Microsoft said, users can recover a quarantined data file by following these instructions:

Close Outlook or Outlook Express.

Click "Change OneCare Settings" in the OneCare main screen.