Sorry to start a (newbie :-/) new post on what is probably out there somewhere but I (and even my supplier) can not find clear instructions for uploading to the Mega2560. Like others who have posted, I have success with the Duemillanove (simple via port and driver) but get in a real mess with the Mega. It seems that the .inf file is to be used not the driver. How do I install the .inf file? Also because the upload error message shows (e.g.) COM 3 already in use, this suggests the software is still trying to use the other method. Would anyone have a link to CLEAR instructions or would like to write some for us all? Also I read that there is an optional separate upload hardware device available (similar to JLink). This apparently avoids use of the boot loader. Anyone using this? Thanks.
Don't know if this is your issue or not, but I had something similar and this is what I found that worked.
Thanks for that. I will try a reload but I feel it's the board firmware on the Atmega8U2 because my system (Windows XP) does not recognise the USB as a valid device. Instead of giving the 'beep' for 'USB recognised' it gives a different 'double beep' sound which suggests that it is trying but finds it invalid. I am having so much success with the Duemilanove and several shields that this has gone on hold for a moment. I will look at how to re-upload the firmware in the 8u2 but also consider building an ISP programmer which could be fun. Just strange there is not much on this yet. Will update soon.
Did you try going to Device Manager and updating the driver for the Arduino device listed there? Point it to the drivers/ directory of the Arduino software.