I have a Plone 2.1.2 site the following issue:
IE7 is prompt to install ActiveX if Office 2003 is installed.

Alexander Limi in his howto Making your existing Plone install look better in Internet Explorer 7 describes the issue

November 7, 2006
Internet Explorer has deprecated the implementation of the XmlHttpRequest object in MSXML 5.0, resulting in security warnings for things like LiveSearch and Kupu — but only if you have Office 2003 installed on the computer where you use Plone (or if you have installed MSXML 5.0 separately). Plone relies on an external library called Sarissa, which needs to be updated to check for the new native implementation of XmlHttpRequest in IE7. It's likely that we will ship a new release of Plone with these included during the coming days, but at the moment there is unfortunately no easy way to fix it yourself.

Here are some comments from Plone-Users mailing list:

Nicklas:

I do not know exactly wich parts of plone that uses it, but kupu is one of the
instaces where you could find this file.

This is the instances i found:

Products/kupu/common/sarissa.js
Products/validation/kupu/common/sarissa.js
Products/validation/CMFPlone/skins/plone_3rdParty/sarissa.js
Products/CMFPlone/skins/plone_3rdParty/sarissa.js

and i changed these lines to this:

    _SARISSA_DOM_PROGID = pickRecentProgID
(["Msxml2.DOMDocument.4.0", "Msxml2.DOMDocument.3.0", "MSXML2.DOMDocument", "MS
XML.DOMDocument", "Microsoft.XMLDOM"], [["SELECT_NODES", 2],["TRANSFORM_NODE", 
2]]);
    _SARISSA_XMLHTTP_PROGID = pickRecentProgID
(["Msxml2.XMLHTTP.4.0", "MSXML2.XMLHTTP.3.0", "MSXML2.XMLHTTP", "Microsoft.XMLH
TTP"], [["XMLHTTP", 4]]);
    _SARISSA_THREADEDDOM_PROGID = pickRecentProgID
(["MSXML2.FreeThreadedDOMDocument.4.0", "MSXML2.FreeThreadedDOMDocument.3.0"]);
    _SARISSA_XSLTEMPLATE_PROGID = pickRecentProgID
(["Msxml2.XSLTemplate.4.0", "MSXML2.XSLTemplate.3.0"], [["XSLTPROC", 2]]);

Duncan Booth:

Have you installed kupu 1.3.9 from plone.org/products/kupu? That should
replace sarissa with an IE7 compatible version.

Now I see Andre Nogueira comment

December 4, 2006
Sarissa team released a new script version
I just put the new version of sarissa.js in my custom folder to solve the script problem.
I still testing, in plone, but looks ok.
http://sourceforge.net/projects/sarissa

So it would be nice if people post comments if they have some issues with new sarissa.js in Kupu 1.3.x or 1.4 beta.