I have to install Agent12c on on Windows Server 2008 R2, already there is Oracle 10g agent installed on the system. The OMS Server is a linux system. I am not able to find the setup of Agent12c for Windows Platform.
Please help in this installation.
In 12C you can get the software using Self Update console (Setup tab > Extensibility > Self Update).
The following document explains how to install agent using response file in silent method: Note 1360083.1 : EM 12c: How to Install EM 12c Agent using Silent Install Method with Response File
By the way, here's some additional information . . .
If you are installing a 12c agent on a platform that is the same as the one on which your 12c OMS is running, then the agent software for that platform is available by default in the software library. So no action is required from your end. However, if you are installing on a platform that is different from the one on which your 12c OMS is running (this seems to be your case), then you must ensure that the agent software for the intended platform is downloaded and stored in the software library using the Self Update Console. Instructions to download the agent software using the Self Update Console are available here. Essentially, having the agent software is a prerequisite.
However, that's a description of only the prerequisite; not really the procedure to install the agent. To install an agent using an interactive console, which I guess is the easiest, read this chapter. It's quite detailed; make sure you meet all the prerequisites before you head to the procedure.
And to install an agent in other ways, for example, using scripts and response files, read this chapter.