You obviously running SQL Server on a cluster.

In that case the service account used to start the windows clustering service must have access to SQL Server. Usually after you installed SQL Server on a cluster this account has access through the Built-in|adminitrators group. When you remove this group you need to add the accountas a new Login in SQL Server.

Start the server in single user mode with minimal configuration and add a new login for the clustering service.

From a command prompt run:

sqlservr.exe -m -f

If it is a named instance you also have to add the -s paramater wih the instance name.

Hope this helps