How to resolve "the specified account already exists" error during SQL Server 2008 R2 installation
First, search the installation log. In my instance it was under the following path: 

C:\Program Files\Microsoft SQL Server\100\Setup Bootstrap\Log\*.*


In that log I found the following entries:

2015-08-07 11:16:50 Slp: Sco: Attempting to open registry subkey 
2015-08-07 11:16:50 Slp: Sco: Attempting to open registry subkey SOFTWARE\Microsoft\VisualStudio\9.0
2015-08-07 11:16:50 Slp: Sco: Attempting to get registry value InstallDir
2015-08-07 11:16:50 Slp: Target package: "D:\shared\~~~~~\sqlncli.msi"
2015-08-07 11:16:51 Slp: MSI Error: 1316 The specified account already exists.


I then located the file sqlncli.msi and opened a command prompt to execute this command:

msiexec /i "d:\[Your sqlncli.msi path]


I then uninstalled the SQL Server Native Client and ran the installation for SQL Server again, success.