Apparently its the second number that gives the detail - 0xC1900101 means it didn't work and 0x40017 is why - if you have the secret code book.
There is a lot of documentation suggesting Nvidia drivers should be up to date as only the most recent support 8.1.
But that didn't work for me - after two failed attempts I removed my gaming mouse SteelSeries software (forget which website suggested it) and all was well and it updated in about an hour.
One final tip. If you have a failed attempt it does not download the whole 3.5Gb again. If you go to the app store and click download 8.1 it skips the download part and then gets on with it.
Its very difficult to imagine whats in the 3.5Gb of download: 'My Computer' is now called 'This PC' and SkyDrive is slightly more integrated than before. Apparently the metro interface is better but I could not see any difference. And with Classic Shell v4.1 you can now turn off seeing 'apps' in the Start Menu so you can almost completely forget your using windows 8 :)