It's a Steam error code, and I searched on google and Steam Support (they don't even mention that error code). I found out a lot of games have that problem on Steam, but I don't know what it means.

When I try to run the game by using the "run as administrator" option on the FalloutNV and FalloutNVLauncher shortcuts, I get two errors, for FalloutNV I get the same Steam error pop-up (Error Code 51) and for FalloutNVLauncher I get a windows error pop-up (Steam Error Application Load Error 3:0000065432).

Here are my specs (if that helps):
Operating System
MS Windows Vista Ultimate 64-bit SP2
CPU
Intel Core i7 920 @ 2.67GHz
Bloomfield 45nm Technology
RAM
3.0GB Triple-Channel DDR3 @ 539MHz (7-8-8-20)
Motherboard
Gigabyte Technology Co., Ltd. EX58-UD5 (Socket 775)
Graphics
SyncMaster @ 1680x1050
ATI Radeon HD 4870 X2 (ATI)
ATI Radeon HD 4870 X2 (ATI)
CrossFire Disabled
Hard Drives
625GB Western Digital WDC WD6400AAKS-22A7B0 ATA Device (Unknown)
Optical Drives
HL-DT-ST DVDRAM GH22NS50 ATA Device
Audio
Realtek High Definition Audio
Anti-Virus
Avast!

I would be grateful if you guys help me out.

i get that when i try to edit the .exe in any way. i think it's a protection thing.

Does installing a couple of textures in the data folder count?

no, that shouldn't do anything, but change your textures.

I'm still going to need help because I can't even start the game. I just installed the game and I haven't touched anything.

Bump, because I still need help fixing this.

Have you tried Verifying the Game Cache?

Now where do I remember saying this exact thing before....

Done that and it didn't work.

I really need your help, please.