Microsoft attempts to keep a regulated patch release schedule — the 2nd Tuesday of each month. However, there are situations when a vulnerability is found, deemed critical, and a patch is released out side of the normal timeline.
Security Advisory 973882, Security Bulletin MS09-034, and Security Bulletin MS09-035 are three such patches that are approved for immediate installation. You should consult with your unit’s IT personnel to determine at what time during the day your computer will reboot as each department may have scheduled this differently, and these updates do require a reboot to complete their installation.
If you have a personal computers on the network that is not managed by an IT unit, you are requested to patch your computer as soon as possible. You are encouraged to patch your home computer as soon as possible, as well. Remaining fully patched is critical to the protection and health of your computer.
Further information, including the technical details, about the patches can be found in this SysNews item