Procmon - little handy tool

Recently, I've faced a problem, when wrong GPO from old misconfigured Windows Server 2003 DC applied to newly installed Windows Server 2008. There was a lot of issues related to wrong registry and file system permissions (the only things that doesn't restore to a previous state after removing GPO), so different services, such as Terminal services, couldn't start and produced a lot of errors. The only tool that helped me was Procmon. It allows to see real-time operations of operating system, check all "Access denied" errors and much more. Now everything works just fine.