Later I have installed the new Client SP4. After this always the Windows Login GUI appears and I have manually to start the Novell client.
What was going wrong during the Client installation?
Novell Client 4.91 for Windows XP is workstation software that brings an easy-to-use, secure, and manageable networking environment to Windows XP and Windows 2003 users. It enables you to access NetWare services from Windows XP workstations or 2003 Windows servers, and tightly integrates either product into your NetWare network. For example, with Novell Client for Windows XP, you can browse through authorized NetWare directories, transfer files, print documents and use advanced NetWare services directly from a Windows XP workstation or Windows Server 2003.