While I can appreciate that Windows can cause an issue, the fact that the programmer is getting some responses that are different to what it expects, leads me to thinking its something other than windows.
# Download the most up-to-date FTDI driverThe latest driver is in 0022, and yes as i stated I tried reflashing the Atmega8u2
# Try a different USB cable (yes I know, but apparently that helped)The cable works just fine in the Duemillanove, and it flashed the 8u2...so it works.
# Try powering the Uno externally as well as by USBCan't exactly see the point in that one
# Put an oscilloscope on the Vcc line in AC mode and check for line noise.The problem as stated many times is that the programe gets the wrong response back.
# Try rebooting Windows in "safe" mode (this may not work, but you could try).Safe mode stops all drivers, its unlikely to be much help.
OK one thing I did notice, is that Windows (and the new inf file is no different), loads usbser.sys.This has a few versions, but is relatively the same through to Service pack 4 in Win2k.The new inf file loads usbser.sys...could this be the mystery windows issue.For the record I changed mine to 5.1.2600.0, with no luck.Many other machines possbly use USB2.0 which uses a different usb driver.
I did note that Bret75 had a problem with USB1, and this may be the problem I'm having.I've run out of time to check it on another machine with USB2.0......
So...the mystery continues, and I'm really glad this wasn't my first experience with Arduino.
I wonder if you are not ruling out in your mind the very thing that might solve it.
You said you had quite a few computers, and it works on most of them.
avrdude: stk500_getparm(): (a) protocol error, expect=0x14, resp=0x41avrdude: stk500_getparm(): (a) protocol error, expect=0x14, resp=0x41avrdude: stk500_initialize(): (a) protocol error, expect=0x14, resp=0x42avrdude: initialization failed, rc=-1 Double check connections and try again, or use -F to override this check.avrdude: stk500_disable(): protocol error, expect=0x14, resp=0x51