... 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.