Please Help in Call of Duty United Offensive

Hello, I had original call of duty and I wanted to try United Offensive.
The problem, the game launches and shows me the menu, but when I click new game and choose difficulty, after I click, the processor loads, the screen freezes and after about 5 mis it throws me back to the main menu. The Quit button doesn't work, the credits button is the same as the new game button, load button doesn't work, and option button works. Any help would be appreciated, thanks.