Most Windows updates are available the 2nd Tuesday of each month. In the trade, it is called “Patch Tuesday.” But, sometimes, when Microsoft decides the threat is too dangerous to wait, they can come at different times as well.
Windows 7 and Vista will check for new updates when you start your computer for the first time in a day. They will be automatically installed at a time that is scheduled in your computer. By default, it is set to 2am, but I routinely change that in my clients’ computers to 6pm (EST).
On Patch Tuesday, it is very important to get the updates that become available, installed as soon as possible. The window of time between when they are first available (about 2pm EST) and the time they are actually installed is the most vulnerable time for your computer.
On that day, sometime after 2pm EST, it is best to start Windows Update (its in your Windows 7 or Vista menu) and click on Check for updates. After it figures it all out, trigger the update process.
Microsoft has been offering a lot of other (other than critical ones) updates lately. Some of them are good for your computer, some of them are of no use to you, and some of them are simply blatant attempts by Microsoft to take more of a share of your computer – e.g., Bing Bar. Some of them will cause your computer harm, e.g. Microsoft Security Essentials, if you already have another antivirus program installed.