Google Update automatically upgrades Chrome, Picassa and other Google apps, installs without asking and is really hard to kill off. It can now be managed, however, from Windows' Group Policy Editor.
Anyone who's done some system or network administration with Windows machines will probably be familiar with the Group Policy Editor, but it works for almost every Windows system, networked or not. The Google Update for Enterprise page explains how to import an administrative template into the Local Group Policy Editor, then either disable it or change the update check interval to whatever you'd like. Wanna get real geeky with it? You can set update policies for different apps, so Google Earth only checks once a month, while Chrome can ping home every week.
We say "almost every Windows system" because the "Home" or "Basic" Windows editions often lack a Group Policy Editor tool. If that's you, hit the via link below for Google Operating System's quick steps to editing your Google Update policies through the registry (which you should, of course, back up before tweaking). Google Update for Enterprise - Help [via Google Operating System]