i tried to upgrade a word press blog from version 3.4 to 4.4 which was running on an iis 7.5 webserver. It turns out that the windows installation of libcurl (used by php) does not have any root CA certificates installed. So the certificate validation fails, and so does the wordpress installation.
so, you add the root CA certificates to your php.ini this way:
open your php.ini file (probably in C:\Program Files (x86)\PHP\v5.4 but you can check in iis by opening a site and using php manager to show you/open the php.ini file).
save this link somewhere under your php installation – http://curl.haxx.se/ca/cacert.pem
add the certificate bundle like so –
[curl]
curl.cainfo="C:\Program Files (x86)\PHP\v5.4\data\libcurl\cacert.pem"
iisreset your server and you should be good to go.