eva2000,
When a process exits it tells it's parent process it is about to die. When the parent acknowledges this death the child's PID is removed from the process table. A zombie process is a process whose resources have been freed, but whose death has not been acknowledged. Zombies are always cleared after a reboot and do not adversely affect system performance. If you consistently get 400+ zombies something is wrong however. Most likely either a program you are using is buggy and not collecting it's children, you have a buggy driver, or something is misconfigured (Unfortunately that doesn't really narrow anything down for you).