You cannot run vCenter on Windows OS as of version 7. Only the virtual appliance.
You mount the ISO to your Windows or Mac workstation, run the installer and point it to an ESXi host. The appliance has to reside on your ESXi Server.
Once you get that deployed and login to the vCenter server, you create a Datacenter, then either add your ESXi host(s) to the Datacenter, or you can create a cluster inside the Datacenter and add your host(s) to the cluster.
This is how it is designed to work. Once you have vCenter up and running with hosts attached, you don't need to login to ESXi host to do any management as it will all be controlled by vCenter.
Nothing will happen to your existing VMs. Once you add your host to vCenter, you will see all the VMs running in the host inventory. Don't worry
I hope this helps. Feel free to ask more questions, I am happy to help you learn
