Win2003 CertSrv application and Vista Clients

We have a Win2003 (SP2)/IIS6 server as a host for an Extranet and we
authenticate users via PKCS#10 certificates issued from the server. The
certificate enrollment pages are customized versions of the standard CertSrv
web pages.

This setup has worked fine for ~5 years with clients running Windows 98
through to Windows XP and browsers from IE4 through to IE7. Enrolling using a
Vista client however results in the browser stalling as it attempts to
download an ActiveX control or displays an error claiming the certificate is
invalid.

http://support.microsoft.com/kb/922706 claims that Longhorn enrollment pages
can be installed on a Win2003 server but then has reduced functionality. Can
anyone explain what "reduced functionality" means in this context as I do not
want to install the Longhorn pages only to alienate our current (non Vista
based) customers (who are the majority) and we are not in a position to have
a separate test server in this environment.

Alternatively, can anyone explain how the proxy server setup should work.

Many Thanks

Shaun