Some news. Even some GOOD news.
I started to uninstall the Jungo driver from within the Device Manager. There was no difficulty.
I then connected the USB cable to the avrisp mkII. This started the Wizard for driver update. I told the Wizard to use this driver : [arduino]\hardware\tools\avr\bin\libusb0
New try to burn the bootloader with avrisp mkII : new issue but this time it concerns the signature of the atmega :
avrdude: Expected signature for ATMEGA328P is 1E 95 0F
Double check chip, or use –F to override this check.
This comes from the fact that avrdude only knows atmega328P and that the signature of atmega328 is 1E 95 14.
Hopefully, I found a very interesting post in the forum :http://arduino.cc/forum/index.php/topic,58670.0.html
explaining how to solve this problem by temporarily changing the signature within the avrdude configuration file.
I found the avrdude.con file here : [arduino]\hardware\tools\avr\etc\
So I replaced the signature with that of atmega328.
New try to burn the bootloader. This time, the led13 was flickering but the process ended with a new error:
avrdude: stk500v2_recv_mk2: error in USB receive
avrdude: verification error, first mismatch at byte 0x10f0
0xff != 0x00
avrdude: verification error; content mismatch
I ignore the real signification of this message but I think I read somewhere in the forum not to pay attention to it.
So I put the avrdude.con back in its original state and … marvellous ! I could again upload a sketch by using the USB to USART converter and the bootloader, exactly as I explained in the first post of this thread.
I then re-installed Studio 4 + the SP3 update and the Jungo driver was there again. I was thus able to verify the signature, the fuses and the lockbits : exactly as they where when I received the atmega.
So, I think the problem was that, for some unknown reason, the bootloader was corrupted and unable to upload the code into the flash memory. The solution was here to re-burn the same bootloader. I think I could have done it from Studio though avrisp mkII by manipulating myself the fuses, burning the bootloader, then resetting the fuses. That will be for the next wreckage !
To Grumps : don’t be desperate, let it to “some” house wifes (sorry for the bad joke. I really don't have anything against the house wifes) . Microcontrollers , AVR and Arduinos are a fantastic world, but not as easy as it is told.
Higher the challenge, higher the satisfaction. I don’t know if that expression exists in English (I speak French) but if it doesn’t, it ought to be invented.
To Madworm : I agree with you. A good programmer is quite useful, if not absolutely necessary. I have no complain about my avrisp mkII , even if I soldered it myself from a cheap kit.