I was setting up SharePoint Portal Server 2003 on one of my home computers. After installation of the Services Packs i went into the administration website to create a new Portal. Before that i made a folder and added a new website to the IIS 6.0 pointing to that folder.
Strange enough the new created website for the Portal was not selectable. Going into the list of virtual servers showed my why.

It seems that some sites and also the one i just made is set to "Not Administrable". After doing a google search (whichs delivered ony 3 hits on this subject) i found out you had to install the service packs. But i already did that. Today i finally found the solution.
Because i'm not always working at home i use a Removable Harddisk which is FAT32 formatted. It seems that putting the folder on a FAT32 formatted disk is causing this problem. Moving it to a NTFS partition or reformatting the FAT32 partition will solve this issue.