shutdown google desktop by right clicking on the tray icon and choosing exit.
start->run and type regedit
In regedit go to My Computer->HKEY_CURRENT_USER->Software->Google->Google Desktop
On the right you will see an entry data_dir. Double click it and change the value data to a directory of your choice.
now start google desktop again and you should see your cache in that directory.
Some words of caution though: