After a failed attempt to download an update to Zone Alarms and System Mechanic, I cannot boot up the computer (not this one). It won't load windows. I am using XP. It gets as far as the stage where it should be loading Windows but all I get is the above message and an instruction to press Ctrl+Alt+Del to restart and when I do, it reappears. Can anyone tell me what NTLDR is and where can I find it. Thank you.