When installing DirSync, the installation fails without a very good explenation. After investigating the MSI logs and the Event Viewer, you find the following statement:
DirSync Installation Fails Product: Microsoft Online Services Sign-in Assistant — Newer version already installed.
The scenario where I have seen this is where a machine is used for DirSync where the Windows Azure Active Directory (WAAD) PowerShell Module have been installed. To be able to run the WAAD PowerShell module and connect it to Office 365, you need to install the Online Services Sign-in Assistant (SIA). That leads you to Microsoft Download Center that provides the SIA, currently in version 2.1. Already have WAAD Module in Place Before deploying DirSync is normal if you are using one of your ADFS STS Machines as DirSync server as well (which is a common scenario).
The problem is that the DirSync.exe installer has the SIA embedded, but with an OLDER version! When trying to install DirSync, it wants to deploy SQL Express, install a stripped version of ILM/MIIS and of course – deploy SIA. This FAILS because you already have a newer version.
Go to the Control Panel\Add Remove Programs and Features and remove your Sign In Assistant. Then deploy DirSync again. It will work perfectly and install the embedded version of SIA. You can later upgrade to SIA 2.1 (or later) again if you like – no worries.