If you're using an older version, you can upgrade by installing the latest version.
'Private Browsing' is a new feature introduced in the latest beta versions of Internet Explorer 8 and Firefox 3. This feature prevents web history from being saved on the computer during Private Browsing sessions.
Only the latest editions of Google Desktop support this new feature. If you're using an earlier version, content accessed during a Private Browsing session may show up in your Google Desktop search results.
We recommend you use the latest version of Google Desktop. If you choose to use Private Browsing with an older version of Desktop, you can still use existing Google Desktop features to temporarily pause indexing during your Private Browsing session, or use Desktop Preferences to disable the indexing of web history.