After installing the 8.1 preview on my Surface RT I had all sorts of problems!
To name a few:
1. Windows updates would fail to install
2. The Windows Store was unavailable
3. Windows failed to activate
In the end I found out that this was all to do with the inability to activate. To resolve this I ran the following command file – http://sdrv.ms/HpMUaD
This adds a valid key and activates the Surface RT. Make sure you follow the included instructions – extract and run as administrator.
Once activated I still had issues with the Windows Store, so to resolve this I did a PC refresh.
After the refresh I was able to install all available updates, and then from the Windows Store I upgraded to the release version of 8.1 (which I might add improves the Surface 10 fold!!)
Any questions just ask. Good luck!
