You should be able to push the agent without knowing the specific OS, as long as it's reported as Unknown Windows.  How did you discover these systems?  Did you discover using network discovery, or Active Directory?  If you discovered using Active Directory, you should have gathered specific OS names like Windows XP and Windows 7.

What fails on the push?  How were these discovered?

@mclemson

Thanks for reply.

When I joined this project, I found both AD Import & Network Discovery were  turned on.

So I can't tell you how were these discovered.

And Error it shows is: No Network Accepted the given path.

Although , Some machines managed to install altiris agent, but now they shows OS name.

Ensure you've met the pre-requisites for installing the agent on XP and/or 7:
http://www.symantec.com/docs/TECH13407