This thread might be an isolated experience but as I have seen others on overseas forums with the same issue I thought I should share my experience:
I have a HP Envy 15 that is a few months old and I upgraded to Win 8.1 when it came out. Overall it worked well and I was a happy user although Bluetooth wasn't working properly. Recently I make one fatal click around the Bluetooth driver area and was greeted with the "sadface blue screen of death" saying "system_thread_exception_not_handled".
The long saga of trying to recover with safe mode, then refresh and finally rebuild started. The instructions were fairly clear - rebuild with Win 8.0, get updates, then update to 8.1, get updates. All seemed well (there was a recovery partition) until Windows said it could not activate 8.0 with an error code saying the product key had been blocked by Microsoft.
First call to HP support got "sorry we don't yet support 8.1 on that model, please stay with 8.0 and watch our website for driver updates". This is 2 months after 8.1 came out so rather slow. The proposed solution to the activation problem was to ship recovery DVDs. I was suspicious that it may be no better than the recovery partition option but HP seemed confident.
Before the DVDs arrived, I tried another call to HP who this time took me through to the phone activation option and said if I could see that then all would be well. It refused the generated code and the nice lady at Microsoft said the code had been blocked and I needed to contact HP.
DVDs arrived from Singapore so rebuilt the PC (needs external DVD player) and of course the same error code appeared. HP support now say a Tier 2 person will contact me. So far they have been helpful but it is a slow process and I have some trouble understanding the verbal instructions from a very quiet phone line from some distant sounding country.
I have now found newer threads which clarifies others have had the same trouble with Bluetooth giving blue screen with a solution in here if anybody is desperate to have 8.1 and get Bluetooth working. However, the main reason for this post is to warn that you might think you have a fallback if the system gets broken but if you have upgraded to 8.1 (I am assuming it is ok if staying at 8.0) then the vendor supplied recovery might not get you an activated system.
Forum messages suggests I am not alone in having trouble with the activation process. With Win 8 the product key is embedded in the BIOS and it seems I have a generic OEM key. One overseas user got so frustrated with support from HP in their country that they purchased a new 8.0 key and are trying to get their money back from HP.
I am hopeful that I am now only a short step away from a fully working system but watch out if you have these same issues. I'll update when there is some progress.