This tip is for Windows SharePoint Services 3.0 and Microsoft Office SharePoint Server 2007.
It happens, server configurations change. If the default file or registry locations for SharePoint change permissions issues can occur on the farm. Luckily there is an easy way to set them back to defaults. Run a command prompt using the SharePoint farm administration account and change directory to the SharePoint binary location. Usually this is located at %CommonProgramFiles%\Microsoft Shared\Web Server Extensions\12\BIN. It is best to run a command to check whether the farm or SharePoint server has a pending upgrade as if your run the next command and there is a pending upgrade, Psconfig.exe will try to finish it, which you may not want to do for various reasons. So first run the Stsadm.exe -o localupgradestatus from tip #1. Then Run psconfig.exe -cmd secureresources to reset the SharePoint file system and registry security settings back to defaults.
Keep in mind if you have changed any of the location of logs/indexes/etc.., that these locations will not be reset. For a list of needed security permissions for WSS 3.0 and MOSS 2007 see the following; http://technet.microsoft.com/en-us/library/cc721662.aspx.