The 64-bit Windows 8 NVidia 296.17 driver for my GTX 580 card off Windows Updates has changed today. I already had the driver installed but today Windows Updates wanted to update it. We'll have to investigate to see if we can tell what changed.
When it first came out it was an Optional Update with driver ID 200000362 located here: http://fg.v4.download.windowsupdate.com/msdownload/update/driver/drvs/2012/03/200000362_07dfffc4d95b942cc145cda2275ad422c9207b5c.cab
Today it is an Important Update with driver ID 4915 and is about 1MB larger in size located here: http://fg.v4.download.windowsupdat