Fighting for the computer with a 4-years old son

How I missed my alone time. That would be not less than 4 years ago! Nowadays the only alone time I can have is in the toilet. I'm trying so hard to earn some income from the net. But everytime I'm sitting in front of the computer my son would be breathing down my neck. After about 10 minutes, he would start whining to get to play on the computer. Not wanting to lose my energy scolding him, I would let him use the PC. He can sit for hours playing games. Usually I try to get the PC back after half an hour but he just wouldn't budge. That's when it start to get on my nerve. After 1 hour, I would just give up and leave him. Sometimes he's not comfortable to be left alone, sometimes he just didn't care. Now my only wish is for another PC so we don't need to share anymore.