>yes exactly that is what i need to do murrayei
>could you describe a little more
If 'B' in my previous description is a single machine, need not route and the A to C issue goes away.
My OpenVPN/Iptables skills are not strong enough to describe it in general terms and has been previously stated, you are better off going through the support channels of OpenVPN, rather than CentOS. If simple firewalling using Iptables is a new or a difficult concept, then you might want to get some hands-on help, as was also suggested.
To answer your original question, though, I wouldn't know where to start to solve restriction of "A to C" using Windows, whereas I am pretty sure you will have all the tools you need under CentOS/OpenVPN... for free.