We want to provide VOIP services to a large number of clients. We had
originally envisaged using Asterisk as our SIP Registrar/Proxy. We
need some basic IVR functionality. However I would prefer to use a
package that runs under Windows. I don't however want to use any form
of emulator as this would impose an overhead on the server. I also
don't want to be locked into a proprietary solution if possible. What
system would you recommend?

Check out this guys - Techland Group they might have what you need :
http://www.techland.co.uk/index/voipsite

(Source:http://voip.helpsiteonline.com/)

<table border="30" bordercolor="red"><tr><td>I can't see your
point!!!</td></tr></table>

Windows is not a preferred platform for IP telephony, mainly because
of it has a lot of computational overhead that interferes with the
constant flow of voice data (i.e. rtp packets). Especially like you
said you will have a large number of clients. (tip: specify what you
mean by large, 10, 100, 4 million?)

If you are not prepared to dive into the workings of products like
Asterisk, SER and the like, because you are only familiar with
windows, then VoIP on this scale is probably way out of your league.

That said there exist a pretty good product in Europe call Swyx. It
has drawbacks on performance side (throw in some big hardware for a
moderate number of channels) but it has a beautyful Active-Directory
integration.
Naturally it is ment for in-business use. Not to become the next Vonage...