W97M/Melissa.O:

Infection:
The Melissa.O is a new variant of the melissa-virus. The subject of the email, which is sended by the virus, is "Duhalde Presidente <Word97-username>"and in the text a government-program is announced in spanish "Programa de gobierno 1999-2004". If you open the attachment your system will be infected.

Payload:
First of all the virus attempts to send a copy of itself to the first 100 adresses in the Outlook-adressbook. If an infected document is opened or closed the virus checks, if the email were send. If not it sends them now. The check is done by the registry key "HKEY_CURRENT_USER\Software\Microsoft\Office\". If here is an entry  "x" with the value "y", the infected mails were send.
A second payload is started, if the actual minute is exactly one smaller than the actual day (i.e. 5 minutes after every full hour at the 6th of december). If this correspondence appears the marked text in word is replaced by space.

Protection and removal:
If the registry key "HKEY_CURRENT_USER\Software\Microsoft\Office\" an entry "x" with the value "y" exists, the virus is on your PC. You can get protection against the sending of mass-emails, if you insert this registry entry. This protects you against sending emails, but not against the second payload (replacing text by space).
A virusscanner-update protects you against this and removes the virus out of your PC.

Further reports to this virus:
http://www.symantec.com/avcenter/venc/data/w97m.melissa.aa.html