I hate to dredge up an older topic (last post a few months ago) but I found a Venue 8 Pro 3845 (the 1GB variant) at a pawn shop yesterday - they simply couldn't get it to power on so they dropped the price way down to just $15+tax and it came with the Folio case and the Bluetooth keyboard too so I took the chance. Sure enough when I got it hope and plugged in a 2A charger for about 30 minutes then pushed and held the power button down about 8 seconds it fired up. It had Windows 10 on it but apparently the previous owner pooched something someplace because it wouldn't finish booting properly.
Anyway, after grabbing the recoveries posted here (thanks to those that shared them), after downloading not only a clean Windows 8.1 ISO from Microsoft using the Media Creation Tool but then also downloading Windows 10's latest build (15063) all I've been able to get running on this device is that Windows 10 latest build. I've made like a dozen attempts to figure out how to force the Windows 8.1 recoveries that were shared to be recognized and they never ever are. I've used diskpart to wipe the internal storage clean of everything and all partitions (even doing a secure wipe on it just in case using "clean all") and still nothing.
So, while I can use Windows 10 on this I'd really prefer not to but if there's no other solution then I'll stick with it. It did activate but oddly enough it shows Windows 10 Home - I read the Dell docs for this tablet and I was pretty sure that every Venue 8 Pro 3000 series tablet came with Windows 8.1 Pro from the factory so this should be showing Windows 10 Pro but alas it doesn't (and yes I did specify Windows 10 Pro with the 15063-relevant Media Creation Tool).
It's tough not having that Dell USB adapter cable thing but I spent an hour scouring various websites like eBay and many others trying to locate one for purchase (even Dell seems to have removed all possible traces of it from their website or maybe I'm just getting stale in my search talents, who knows). I was able to use a 4-port USB hub (non-powered) to get at least a mouse working enough to get Windows 10 running and then install the necessary Atom chipset drivers to get the touch functionality working.
I've read that the Windows 8.1 Pro ISO should automagically recognize that it's being installed on an OEM device that has the Product Key embedded in the BIOS but it's simply not working. Once I got Windows 10 installed I use a tool called ShowKeyPlus to pull the embedded Product Key and now I have it - the tool also says the OS is Windows 8.1 RTM CoreConnected which is not something I've ever heard of before personally, that could be a reason why I'm having issues I suppose.
I did locate that CoreConnected ISO and it's currently en route to my laptop so, perhaps that will work to get Windows 8.1 installed and I'll see how it goes from there, I have the 8.1 drivers from Dell already downloaded and can install them after the OS is running, not going into all the trouble of slipstreaming them into the install.wim at this point.
Anyway, worse comes to worse I can leave Windows 10 on it, and the Bluetooth keyboard is pretty cool I suppose but I have large hands so it's difficult to make use of it. I have a Dell Bluetooth Travel Mouse I found in a thrift store for $4 a few months ago, works fine so I suppose I have a complete Dell tablet package now, just wish I could find that USB adapter cable they had someplace for a decent price (meaning not $30+ which is insane).
Thanks to the contributors that have shared their recoveries and their knowledge, it's truly been helpful to find this thread.