The Security Mentor

Most people I know use their email program as an archive or a database, so they can search for that one piece of information from a year and a half ago.

Which means that when spam comes in with a toxic attachment, you want your antivirus software to react against that one message and not the entire email storage.

According to several reports including one from a Microsoft MVP, Windows Live OneCare has placed entire mailboxes into quarantine when an infected message comes in. It can take some hunting to retrieve the email. Some people claimed that years of their email had been deleted, but it seems possible that they simply couldn't find the quarantine area (some people said it's in a hidden directory).

You can exclude your Outlook (.PST) or Outlook Express (.DBX) files from antivirus scanning, which strikes me as a lousy idea but might be safer.

Offhand I didn't see a mention of Microsoft fixing this, though by now they certainly might have.

UPDATE 3/16:

Microsoft has issued a fix, and their European business security manager has said of OneCare "...they shouldn't have rolled it out when they did".