Before we get to that stage I did find something else ..
Under System > Settings > Appearance > Template I found that the entry read "Template 2" despite the Extension not having installed properly (I used your built in install process) and having been deleted. Changing this to "Default" seems to have cured the problem and also cured the problem of some error messages refusing to depart this life despite several attempts to delete them via the System > Messages screen.
The only conclusion I can reach is that, on a windows server at least, installing extensions is a risky business; and uninstalling them even riskier. However, a brief bit of tinkering with the css files indicates I can customise the appearance of the shop site sufficiently and, by replacing graphic files, adjust the parts that the css doesn't reach.
Abantecart has said, in a different thread, that you don't 100% guarantee that the system will work in every respect on a windows server. I do think, in your own interests and that of your reputation, that you need to test the system yourselves on a windows platform by attempting all the system functions as a user and verifying they work. By doing this yourselves you will be able to dive 'behind the scenes' as necessary to resolve any issues that reveal and then include them in the next available upgrade.
Whilst I don't have time to do this for you I will be talking to my hosting company to see what is involved in putting this system in a HTTPS location and I will let you know of any more problems that surface. The 'clean installs' will then benefit form what I have learnt from the two threads I have so far started.
Thank you for your help.