Summary: I could not get the card to work properly on Windows 8.1 x64. Intel has stated it is only offering "In Box Support" for this card - not full support - which means drivers downloaded from their site will *not* work under Windows 8.1. (Windows 8.0 is okay.) Even manually downloading the drivers and point Windows at them will not work. (Search for "Is my Intel Ethernet Adapter supported in Microsoft Windows 8.1".
Summary: The headline says it all. Windows XP gave constant error messages. Windows 7 had delays in all phases of networking but no error messages. Replaced it with an older card from an obsolete PC and now have a fast connection.