I'm having an issue with uploading sketches to my Arduino Uno. In the IDE, it sees that the Arduino is plugged into COM4, and the same shows up in Device Manager (I'm running XP). The Arduino is lit up nice and green. Yet when I try to upload, I get:
avrdude: ser_open(): can't open device "\.\COM4": The system cannot find the file specified.
I've tried going into Device Manager and changing the COM port number assigned to the Arduino, which changes the port number in the IDE, but I always get the same error no matter which one I choose.
This has been happening for weeks now, and I used to be able to fix it by some arcane combination of unplugging and resetting the board. But now, none of this seems to work - I'm totally locked out of the Arduino!
I've been combing through Google searches of my error and it hasn't helped. Any help would be much appreciated.