@zekepots 

Hi I had similar problems installing TD781nd v3 on an HP elitedesk 800 sff g3 pc running Windows 10.

Symptoms

Installed card: device manager would not even detect the card

Installed driver: driver on cd supllied was not correct (not eu version device manager shows a tplink card but 'not active' or similar) but managed to download from tplink site. (I used an usb wifi card but you coild connect directly to router of needed )

Now card detected but card details in device manager gave "error 10".

Tried changing slot, removing graphics card but same issue. Actually got quite frightening as sometimes pc would not boot at all or gave blue screen. Removed card, tried rebooting a few times and got system up. 

Note: green light at back was constantly off.

I did many things but finally it workes ao I list them here. 

Went to hp site and download software download manager.

In parallel windows update installed some Installs but I believe irrelevant.

Downloaded many HP drivers amongst which various intel firmware and chipset updates and BIOS UPDATE.

BIOS update failed a few times but worked in the end.

I then switched off and reinserted tplink card and it works. Green light flashes slowly when not connected and faster when connected.

I hope this helps