Hi,
The S60 seems, or rather mine seems to be REALLY sensitive to low battery power.
One thing to maybe try is...Plug power wart into power port...WITH battery switch OFF...and with the USB cable plugged into both PC and S60, switch the battery back to ON...see if the unit will now go into bootloader mode...
That will make sure you have plenty of power when the unit begins to power up from battery switch off position....
Dave