It seems that KB948109 has caused some trouble for many people. On my Windows Server 2003 machine the installation failed with error code 1920:

MSP Error: 1920  Service 'SQL Server VSS Writer' (SQLWriter) failed to start.  Verify that you have sufficient privileges to start system services.

I checked the SQL Server related services and found that the SQLWriter service and other services were disabled.

This wasn't really surprising, given that I had disabled all services I don't use. However it caused the installation to fail, becausen the installer wasn't able to start the disabled services.

So I set the service startup type to manual, manually re-ran the installer and everything worked out fine. After installation I disabled the unneeded services again.