The best way to check whether or not you need to install the update is to check Windows Update (WU) yourself. When you manually run WU, it will show you at a glance whether or not your system is up to date. If it is, then on the right hand side it will have 'Windows is up to date' in a green banner together with a green shield with a tick in it. To check for individual updates, click on 'View update history' in the left hand panel. All updates that have been installed since you installed Vista will be listed here. The default sort order is by date, with the most recent at the top of the list. To make things easier, click on 'Name' to have the list sorted alphabetically by name. You can then scroll down the list to see if KB942615 is installed, as well as the others listed in the bulletin. On my system, KB942615 is listed, together with other updates, as being installed on 01/01/2008, although it was probably released earlier. This is because I had only just reinstalled Vista. If you have installed SP1, then this update would have been included with that as SP1 has brought together all important and recommended updates that have been made available since Vista was released. I am considering signing up for the notifications as well. It is important to realise that the notifications apply to all editions of Vista, and that some of them might not necessarily apply to your system. If you want to check on the update status of your system, then there are a couple of programs which might help you. They are the Belarc Advisor and Secunia PSI (links given below). Belarc checks your update status whilst Secunia checks your applications status. Both programs need to be run with administrative credentials. When you run Belarc, you are only interested in the 'System Security Status'. Both 'Virus Protection' and 'Microsoft Security Updates' need to be shown with a green tick. The other category does not apply to Vista. If either category shows red, then you will need to investigate that category by clicking on it. Secunia is fairly self-explanatory. Any vulnerabilities it exposes can be corrected by going to the relevant website(s) for the program(s) concerned and updating the program(s) from there.
Dwarf
Belarc Advisor - Free Personal PC Audit Vulnerability and Virus Information - Secunia
Click on 'Personal (PSI)' under 'Software Inspectors'.