EP installation error

Dear Sir,

Whenever I am starting the Installation of EP(OS - Windows, Database - MS SQL) through the SAP installation master cd, it gives me error "CJS_20003 while adding instance DVEBMGS02:an instance already exists".

On my server 3 Instance is already running so adding another instance on the same server I am getting the above mentioned error.

Please Rply me soon.

Thanks in Advance.

Rohit