Secunia

Microsoft OutLook Express suffers a vulnerability allowing attackers to construct malicious S/MIME e-mails that may execute arbitrary code when viewed.

This vulnerability gives the attacker the rights of the logged in user. This vulnerability has the potential to be abused by malware like viruses.

This is exploitable regardless of the security settings in Outlook Express.

Solution
Microsoft has issued patches which should be applied as soon as possible:
Further details available in Customer Area

Deep Links
Links available in Customer Area