Games won't start after installing a new vga

amon47

Honorable
Dec 6, 2013
10
0
10,510
Hi !
I've just bought a new vga (Radeon HD 7850). i had GT 240 before.
Everything was fine, i downloaded the drivers, played some games and everything was ok.
After testing it, i decided to uninstall the Nvidia drivers because it seems to me that i didn't need them anymore. i did it using the programs and features in Control Panel. after restarting the PC, the 2 games i tried didn't start.
Assassin's creed IV Black Flag: when i tried to start it, nothing happend. and nothing changed in task manager.
Battlefield 4: It says that i need directx 10, which is weird cause i have the newest directx installed on my PC.

Also, KMplayer won't work... it doesn't show any video but plays the sound, VLC and jetaudio work just fine

System Information:
OS: Windows 7 Ultimate SP1
Processor: Intel Core i3 CPU
Motherboard: J&W JW-H55T-PRO
RAM: 8 GB
VGA: Radeon HD 7850

So... What should i do?
Thanks in advance