Geoff, I don't use a Windows server, but I thought that I would share with you the php extension that I turn on for my GoDaddy hosted server. Yes, I know that this is a completely different environment, but sometimes a little insight might help. You will see that I just have to click a button to turn mysqli on. The other extensions that I turn on also show on this page. Good luck.
Installing and upgrading help
Upgrading php for 2.7 moodle windows 2007 r2
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