- In Outlook 2007, go to "Tools" > "Trust Center"
- Click "Add-ins" on the left-hand side of the resulting window
- In the lower part of the window, change "Manage: COM Add-ins" to
"Manage: Exchange Client Extensions"
- Click "Go..."
- Uncheck the "Google Desktop Search Outlook Add-in" then click "OK"

This leaves the Google Search Outlook toolbar available, so you can
continue to use it to search for messages. The only downside is new
messages won't be indexed until you turn this add-in back on. So I
just temporarily disable it on the rare occasions when I have to open
a forwarded message.