Thank you for posting your question on the Mellanox Community.
Unfortunately, the Mellanox Infinihost III adapter MHGA28-XTC is EOL for awhile. Our latest drivers do not provide any support for this HCA.
I have found the following post online, regarding the bring-up of these HCA's under Windows 7.
The link to the post -> http://andy-malakov.blogspot.com/2015/03/connecting-two-windows-7-computers-with.html
Hopefully the post's instructions will resolve your issue.
Thanks and regards,
~Mellanox Technical Support