When you just created your SharePoint Shared Services Provider (SSP) and are trying to navigate to the admin page through Central Administration you can get prompted for credentials...  The problem is, even using the Farm adminstrator credentials do not work and it results in a HTTP 401.1 unauthorized error.  Not to worry,  the fix is easy.  The issue is a default security setting in Windows 2003.  Open up your registry editor (regedit.exe) and go to HKEY_LOCAL_MACHINESYSTEMCurrentControlSetControlLsa.  Add a new DWORD value called "DisableLoopbackCheck" and set the value to 1.  Close out of the registry editor and restart IIS.  Go back to your SSP admin site and you should be able to login now.