When you try and create or update a crawl schedule, you are prompted for a username and password, but for some strange reason your user credentials doesn't seem to work.
Upon examing the logs, you will find an Access Denied error.
This error occurs when the WSS_WPG group does not have the correct permissions to the c:\windows\tasks folder.
To solve this,
Open up the command prompt, type attrib –s %windir%\tasks, navigate to the c:\windows\tasks folder using windows explorer, right click and select the properties and then the securities tab, grant the WSS_WPG user Read, Write permissions to the folder, now reset IIS, using the following command, IISRESET \NOFORCE, then reset the folder back to the default view using the following command in the command prompt attrib +s %windir%\tasks.
For more info please see Microsoft article http://support.microsoft.com/kb/926959