Computer won't boot-up

I'm a newbie. Had a local computer shop load some software onto my new computer. When I got it home and hooked up the computer for the first time at home ( -triple checked all connections to make sure they were correct) and powered the system up ( UPS first, monitor second and finally computer )the computer wouldn't boot-up. The computer and monitor are both getting power but the Windows Vista Home screen doesn't come up. The computer shop claims they experienced no problems. What am I doing wrong? Had to manually shut system down -Tomm.