Safe mode, starts Windows in a limited state, where only the bare essentials are started. If a problem doesn't reappear when you start in safe mode, you can eliminate the default settings and basic device drivers and services as possible causes
A clean boot helps eliminate software conflicts. Article below describes how to troubleshoot problems in Windows 7 by performing a clean boot. This article also describes how to start the Windows Installer service and how to reset the computer so that
it starts as usual. To configure your computer to clean boot refer the following link.
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7