I don't recommend that users use this tool unless they are instructed to do so by a Norton malware removal specialist.
The NPE will often flag legitimate software for removal (i.e., false positive detections) and has been known to damage Windows operating files during remediation of rootkits / bootkits and other malware that can hook itself deep into the Windows kernel.
This tool is a very aggressive scanner that is designed to be used as a rescue tool in emergency situations when your operating system becomes unstable or you believe you have deeply embedded malware that cannot be detected by a standard antivirus / anti-malware scan. Please read my post about the dangers of using the NPE. If you are happy using Norton 360 for real-time antivirus protection on your Win 7 SP1 machine I would suggest that you go ahead and renew your subscription and worry about the Norton Power Eraser (NPE) error later.