As covered in the pre-announcement, Microsoft released two bulletins today:

MS06-012: Critical Vulnerability in Microsoft Office, KB905413

This update fixes a number of different Excel vulnerabilities, and a "Malformed Routing Slip" vulnerability which affects muliple Office components.

All the vulnerabilities come down to the same issue: If you open a malformed file, an attacker could get control of the system as the user opening the file.

If you use Microsoft Office, you should apply this patch quickly.

UPDATE: 2006-03-15: PoC exploits have been released.  The patch window is closing rapidly.

MS06-011: Priviledge Escalation in Windows (Important)

It may be possible for a regular user to obtain the privileges assigned to a service. A lower privileged user could change the configuration for a service in order to have it execute code or modify the system in other ways, once the service is running at the higher privilege (e.g. 'system').

This vulnerability has been disclosed for a while now. It is important to note that a "service" is not just a "server". Services typically have to run at a higher privilege level as they require access to files across multiple users, and access to system resources.