Maybe my experiences can help you out a little Fire488. I've installed Fallout 3 several times, and had issues with each install in various ways. I'll briefly chronicle what I went through and how I fixed it.

Vista PC - Install 1:

I purchased Fallout 3 from Steam and installed it on my 32-bit Vista Ultimate machine with UAC enabled. Once the download completed, I launched the game from within the Steam UI, at which point it requested UAC permission to continue. It went through what looked like several program installs, including Games for Windows Live, .net FX 3.0 and Visual C++ redistributable.

After this, it launched the F3 launcher splash window. My "Play" and "Options" choices were grayed out as if the game was never installed. The launcher threw an error dialog which said certain key components of the game were missing and to try re-installing from the retail DVD. (What the hell? I don't have a retail DVD. Steam purchases FTW )

I tried to get around this by launching fallout3.exe from the Steam program files directory directly, whereupon it hung for a few seconds and gave me the same spiel about re-installing from DVD. I tried running fallout3.exe directly as Administrator and allowed the action in UAC, whereupon it threw the "Application load error P:0000065432" error dialog. Thinking maybe this was caused by running it outside of Steam, I switched the program compatibility to always run fallout3.exe as Administrator and applied the change.

When I tried to launch Fallout 3 with this changed setting from within the Steam UI, the same "Application load error P:0000065432" error dialog appeared, then Steam reported that "This game is not available at this time. Please try again later." Again, UAC prompted me for permission to continue and I approved this action.

Vista PC - Install 2:

A bit out of sorts about this issue, and never having had a quick turnaround by Valve support or Bethesda in the past when I've had issues, I borrowed the retail DVD of Fallout 3 from a friend to see if installing from DVD would do the trick. I installed it to the Windows program files directory (not the Steam directory) and it launched just fine, running as Administrator or a normal user. This worked with UAC running.

After playing the game for a few minutes, drooling over it, and wanting it to run normally as I legally purchased it, I decided to try my own troubleshooting of the UAC / Fallout 3 installation interaction and processes.

I found out via UAC (it prompted me to approve the launching of Fallout 3, then I clicked "Details") that there are several script files in your Steam Fallout 3 folder that appear to be generated upon purchase of the game and uniquely tied to your Steam UID. These files are not present in a retail installation AFAIK.

installscript.vdf

This file appears to have been created by Valve to aid in the installation process of the game. Normally, when launched from a retail DVD, the installer copies all files to the installation location on your HDD, then creates a score of entries in your system registry. Because Valve doesn't like installers to run after you purchase and download a game from Steam, they created this script that replicates what the setup.exe installer does, namely create all the registry entries for the game to run properly.

runprocess.vdf

This script runs on the launch of Fallout 3 from the Steam UI and prompts the installation of Games for Windows - Live and .net FX 3.0 if they haven't been properly initiated. This contains the parameters that the retail DVD setup.exe uses to install these pieces of software.

Vista PC - Resolution:

I went into "Turn Windows Features On/Off" in the Programs area of Control Panel and check all items under ".net 3.0" which needed a reboot. After rebooting, I set the Steam executable to always run as Administrator and applied the setting. Upon launching Steam, I allowed the action in UAC and tried running Fallout 3 again.

It went through the "first time" launch script again, and installed .net FX 3.0, GfW Live and C++ redist perfectly, then brought up the Fallout 3 launcher splash screen. The "Play" and "Options" items were selectable! I clicked "Play" and voila, no more "Application load error P:0000065432"! The game launched perfectly.

Vista PC - Summary:

You must be running the Steam.exe program as Administrator in order to properly run the runprocess.vdf that installs all program dependencies. Installing the dependencies separately did NOT work - they had to be run from the script, as the runprocess.vdf file passes along extra parameters to the install process which REQUIRE Administrative access and to be allowed in UAC, if applicable.

Be sure you have all .net framework 3.0 items installed under "Turn Windows Features On/Off".

I have another install on Windows Server 2008 Enterprise, but that went off without a hitch in Steam. I'm thinking it's because UAC isn't an option in Server 2008 proper. Games for Windows Live doesn't work in Fallout 3 under Server 2008, but that's a whole different story for a whole different thread.

It's unfortunate, but I think I may agree with Bethesda not supporting the Steam distributed product with regards to installation and launch issues because of Valve's custom install scripts. It looks like Valve has modified the game's installation beyond what Bethesda originally supplied them with and it's causing issues on some machines. Valve should absolutely be providing product support for this problem, and for this product in general.

I hope some of the above will help you get the game running and defeat the "Application load error P:0000065432" error. Let me know!

EDIT: I know you're on 64-bit Vista and I'm on 32-bit, but the similarities between your problem and mine were too strikingly similar to be coincidental (at least IMO) so I thought I'd try to help you out a bit.