Are you sure that COM1 is what is being assigned to the Mega? There are two things you could check here:
1. Open the device manager (with the board plugged in), and check the Ports section to see if you can see the Arduino Mega. Check what COM was assigned.
2. With the Mega plugged in, open the Arduino IDE and select the Mega board and see if it auto detects it from there.
You mentioned that you had installed Repetier Host, did you also install Repetier Server? R-Server could be controlling the port and not allowing access from anything else. If this is the case, kill the process in the task manager and try reconnecting in Repetier Host.