I need a server redundancy software?

Hi,

I have an active directory server with some installed services on it..

I want a software that enable me to set a redundant server.. I want 2 servers 1 master and 1 slave, that in case of failure the slave become the master automatically.. and it should be identical from the first server.

I need more than one suggestion.. then I will compare the prices and the effectiveness and decide.

Thank you very much, any help will be appreciated.