Google Desktop added so many independent features that the application should include some profiles you can select when you install the application.
1. Desktop search
If you don't want the sidebar, gadgets and other distractions, just right-click on system tray icon and select "Deskbar", "Floating deskbar", or "None", depending on the position of the search box.
If you only want the sidebar, but not the desktop search functionality, go to settings and check "Disable indexing of new items".
3. Application launcher
Google Desktop indexes the shortcuts from the Start Menu, the services from Control Panel. To use Google Desktop only as a searchable Start Menu, disable all the options from "Search types". You should also select "Launch programs/files by default" in the "Quick find" section. Then press Ctrl twice and enter the first letters of the program in the quick search box.
4. Web history search
Google Desktop indexes all the web pages you visit with Internet Explorer and Firefox. By itself, this is a very useful feature, so if you only want an enhanced web history, unselect everything except "web history" in the "search types" sections from the preferences page.
Another way to disable indexing, the sidebar or both is to go to Start Menu and click on "Uninstall Google Desktop" (or press Ctrl+Ctrl and type "Uninstall Google Desktop"). This works only if you have the latest version (Google Desktop 5), which is now out of beta and available in 29 languages.