Installing Windows remotely is beyond the capabilities of RAdmin. Indeed, it is beyond the capabilities of any remote control software, because software needs a running operating system in order to function.
In order to install Windows remotely, you need to be able to control the computer before it has booted the operating system - pressing a key to get into BIOS, boot from CD, etc. The only way I know of to do this is with a networkable KVM switch, such as those offered by Raritan. Here is a link to Raritan's "KVM over IP" selection: http://www.kvms.com/kvm-over-ip.asp
As you can see, those are somewhat pricey. But they do allow full control over a remote computer, even in the BIOS or a pre-boot environment.
(Note: I have not done any research into competing products; I only mentioned Raritan because I have heard of their product line; I have not actually used any of their products despite my efforts to get my employer to invest in one! )
Hope that answers your question.
EDIT: I have to correct myself; the link above is NOT from Raritan's site, but rather a KVM shopping site that lists KVM's from several vendors, including Raritan. (That's what I get for trying to rush this in between assignments at work!) ops:
Another EDIT: For you technically savvy network folks out there, I deliberately left the topic of RIS out of the discussion, because I felt that its complexity and only marginal relevance made it not worth the time and effort to explain it.