If you installed something that says it is supposed to make PhysX run, get rid of it. Even if it is legit it's an Nvidia driver and wouldn't be compatible with your card.
Some games, Metro 2033 for example, require PhysX to be installed or won't run, even if you don't have an NVidia card. Why this is, I don't know. You can check Programs & Feature, and if you've never owned or installed any NVidia products but see it there, there's a 100 percent chance a game installed it because it's required.
Just to double check this, I uninstalled it from my system, then attempted to run Metro 2033; The first thing the game did was install PhysX before running. This is the Steam version. I'd imagine if couldn't download it, it'd complain and wouldn't run until installed. So, yeah, some games require it.
Anyway I swore Witcher 2 also required it but... Anyway if it's needed the game will (should) install it. But if i's needed, and for some reason you don't have it, it can be gotten here -
NVIDIA DRIVERS 9.13.0725 (PhysX Software)
Peace
I stand corrected. I've had it for so long I just forgot about it. Still, PhysX should not cause any problems unless your CPU is seriously underpowered. The Intel Extreme series CPUs are more than enough to handle PhysX on any game, especially with a 7970 GPU backing it up. If other games are running fine, there's no reason Witcher 2 should be running like it is on low settings.
At any rate, PhysX is not the cause of Witcher 2's issues. Witcher 2 does not use PhysX, it runs on Havoc. There's talk that Witcher 3 will use PhysX, but 2 is optimized for AMD cards. In fact, it was given away as one of the free games that came with an AMD card purchase not too long ago.
I suggest trying running an adware, malware, and virus scan. Adware or malware can both cause issues with games. You need some sort of anti-malware program aside from whatever your Virus protection is. Most Virus scanners don't get rid of adware and malware, just viruses. Even if yours does, it's a good idea to have a secondary program for it as one will often find things the other misses. I recommend Malwarebytes, it's free and works well for cleaning out adware and malware.
Also, try turning off background programs. Go to your system tray and turn off any extra programs running in the background. Try going through your Program Files and see if you've maybe picked up some extra toolbars or other junk programs that might be running in your background. It's a good idea to scan through them on occasion just in case something slipped past you when you were downloading something. Some programs like to dump extra little toolbars and trackers into their installers. It's never a good idea to just blow through an install as they'll often try to make these installs look like part of the program installation.
You can also try [temporarily] disabling firewalls and antivirus. Either can interfere with games sometimes. You can add exceptions and get around it if either is causing the problem, but the easiest way to find out is to disable them and try running the game. Again, -do not leave them off even if they are causing the issue-. Just use the settings in either program to let the game through so it can work properly if it is the issue.
You can also try disabling scaling in the CCC settings if you have it turned on. I know it's weird but I've had scaling cause issues with some programs in the past. Probably not the case.
Another possible [but unlikely] fix is the AMD gaming Evolved app. You can get it from the AMD site and it does improve performance slightly for AMD GPUs. It is also free.
It opens a window and runs in the background, but it does optimize a lot of games to run better on AMD GPUs. It 'rewards' you for playing games with some sort of points system I've honestly not paid much attention too and if you like you can use it to enter a drawing for hardware or game giveaways if you want.
It's similar to the Steam Client in some ways and has some social features and a ranking system. It also keeps track of how long you play and lets you know how long you've been playing. I do kind of like having it tell me how long I've been playing when I close a game. That stuff is easily ignored if you're not interested and I only use it myself because it does actually optimize a lot of my games to run slightly better.
Not sure what else might help.