Hello, I'm having this same problem except I can't upload blink... seriously, blink!
I'm using windows 7 and arduino 1.0.3. I've tried uploading blink onto my rambo (mega2560) and another ordinary mega2560 and neither will complete the upload.
I then checked the troubleshooting guide in the Arduino help and followed the advice.
In my case I noticed that whether or not I had the arduino plugged in, COM1 was still available in the serial ports. From there I went to Control Panel --> Hardware and Sound --> Device Manager. In there I found that the mega boards were popping up as unknown devices so I pointed win7 at the driver directory in C:/Program Files/Arduino 1.0.3/Drivers and voila... blink uploaded.
The answer, at least for me, was to follow the Arduino troubleshooting guide found in the help menu of my arduino IDE.