Can I program the Arduino with an RS232 to USB cable ? Is it possible ? And how should I connect them ? I'm asking this since I got my Arduino unregonized by PC : http://arduino.cc/forum/index.php/topic,123119.0.html
I saw this but its was totally unclear how to wire it up http://solutions-cubed.com/programming-the-arduino-uno-with-a-serial-port/
That circuit uses a Level converter chip to convert from RS232 levels to TTL levels.
You could just as easily buy one of these
http://www.ebay.co.uk/itm/1x-USB-2-0-TO-RS232-Converter-UART-TTL-Serial-CP2102-For-Arduino-UNO-ATMEGA-PIC-/320923619625?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item4ab889d129
and connect it to the Tx and Rx pins as shown in the listing, not sure about the reset circuit, you may have to do that too.
At the end of the day, its a messy hack really, cut your losses and salvage the processor for other projects or make your own Duino on stripboard or breadboard, its easy to do.
Thank you 
seems a good cheap idea but I already have a usb to serial DB9 cable ... I can't use that ?
... but I already have a usb to serial DB9 cable ... I can't use that ?
Probably not... Standard RS-232 uses a higher voltage that swings positive and negative (12-15V), and you might have to fake-out (or loop-back) the RS-232 handshaking.
I agree it's just best to just cut your losses.
I had an Arduino that wouldn't program after awhile. (It was "working" and running a "blink LED" program), so I bought an [u]AVR ISP[/u] programmer. It didn't work either.
You could use a MAX232 to convert the RS232 signal to the digital signal that the Arduino is looking for.