Single Sign On upgrade issue 5.1 to 5.5

Hi,

Has anyone encountered the below error when trying to update SSO to 5.5? I have downloaded the latest install package 5.5 update 1 but I keep getting the error. The server is a 2008 SP2 machine.

Product: vCenter Single Sign-On -- Error 1923. Service 'VMware Directory Service' (VMWareDirectoryService) could not be installed. Verify that you have sufficient privileges to install system services.

The things I have tried so far are below after reading various things online.

I have tried using different domain admin accounts

Rebooting the server

Starting the Windows Error Reporting service

Disabling the VMware Converter Services

Disabling the AV

I have also seen the article below but which has a similar error that mentions certain ports should be free, however version 5.1 was installed without any issues and has been working for months.

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2069695

Any help would be appreciated.

Thanks,

Graham