missing/corrupt windows file - XP Home Edition

Hi

My system on awakening from stand by showed the message: cannot open windows - missing or corrupt file windows\system32\config\system. It will not allow me to use the repair facility on the installation CD, neither will it allow me to reinstall XP - advises to boot in safe mode but it will not allow me to do that either. I am desperate to recover non backed up data files but even if I wanted to format and start over I could not as I can not get into a command prompt even - I am tearing my hair out - please can anyone help?! I can get an a: prompt with a startup disk but if I try to change drive to c: I get an invalid drive message.

Thanks - Cheryl