How can I back up my personal search index?
Your Google Desktop personal search index is located at C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\Google\Google Desktop.
To back up your index, first turn off Google Desktop (right-click the icon in your system tray and choose "Exit."). Then copy this entire folder to your usual backup location (most likely a CD, an external drive, or something similar).
To restore your index using this backup file, turn Google Desktop off and replace the C:\Documents and Settings\%USERNAME%\Local Settings\Application Data\Google\Google Desktop folder with the one you saved. Then turn Google Desktop back on by going to "Start" > "Programs" > "Google Desktop" > "Google Desktop."