START UP PROBLEMS

I am trying to assist a friend. He has Windows XP Home installed on his PC. After choosing one of the two operating systems from the operating systems choice menu on his PC, the computer reverts back to the choice menu. However, it starts the other one effortlessly. Both are installed on the C: drive. Also, how can I delete this extra OS, since it seems like it is the same one which was re-installed incorrectly. I’ve tried to start the OS in safe mode,but it does the same thing.