Ok, this is going to be a bit difficult to explain, but here goes:

Originally, I was the proud owner of the 1C/Cenega version of the game, witch ran perfectly for me.

A few weeks ago, someone I know from the RoW territory gifted me FNV (it was on that great id software steam sale, I think).

Steam views these 2 versions of the game as seperate entities entirely, therefore not preventing such a gift being recieved on my end.

Well, I wouldn't really mind having them both on my library, except for the fact that my 1C verison will no longer run, giving me a constant and permanent "This game is currently unavailable" error. At the same time, the RoW version will not launch at all - it loads the game launcher, but when I try to start the game, the following error occurs after the small screen in the corner becomes black screen, but before the intro:

FalloutNV.exe has stopped working

Problem signature:
Problem Event Name: APPCRASH
Application Name: FalloutNV.exe
Application Version: 1.4.0.525
Application Timestamp: 4e0d50ed
Fault Module Name: FalloutNV.exe
Fault Module Version: 1.4.0.525
Fault Module Timestamp: 4e0d50ed
Exception Code: c0000005
Exception Offset: 006087e5
OS Version: 6.1.7601.2.1.0.256.1
Locale ID: 1049
Additional Information 1: 0a9e
Additional Information 2: 0a9e372d3b4ad19135b953a78882e789
Additional Information 3: 0a9e
Additional Information 4: 0a9e372d3b4ad19135b953a78882e789

I have tried a great ammount of things that could have possibly fixed the problem, I'll try to list everything, but I may miss something in the process.

I have done the following:
Tried launching it on Win XP x32, Win 7 x32 and Win 7 x64
Reinstalling the game (obviously)
Reinstalling the OC (as stated above)
Reinstalling Steam
Applying the DLL fix
Running with steam community set to off
Running as administrator and in Win XP SP3 compatibility mod (for 7)
Installing latest drivers for everything
Installing beta or older drivers for everything
Turning off/deleting codec packs
Installing the game on a clean system with nothing but drivers and steam on it
Running the game with steam off
Running the game from the game directory directly
Changing the language
Defraging the game cache
Using various combinations of all of the above

Just FYI, Fallout 3 (Steam) runs perfectly, the problem occurs only in both instances of FNV.
I have read hundreds of pages of information on the subject of FNV not launching, and no solution has worked for me.

I would really appretiate some help with this mess, seing as Lonesome Road is soon to be released, so thanks in advance

-MIB2

EDIT:
My specs:

Intel Core 2 Duo 2.4 GHz
2 GB RAM
Nvidia GeForce GTX 460 2GB
Creative Sound Blaster X-fi Elite Pro
Win 7 Ultimate x64