First step is always simple. Here, you will have to open up a program called "Regedit" that is already automatically installed on your OS.

To find it, simply press your Windows button, and type in "Regedit". Only one program should be a result, you need to click on it and open it up.

Now that you have Regedit open, it's now time to find the folder that your Assassin's Creed III is installed on.

To do this, you will need to navigate through many folders and find the correct one.

Start by clicking the arrow next to the "HKEY_LOCAL_MACHINE" folder in order to open its subcontents. Then you have to do the same for a folder called "SOFTWARE". Afterwards find the folder "Wow6432Node" and open its subcontents. From this point it should be quite easy to navigate to the AC3 folder. You need to find folder called "Valve" to access its subcontents, then "Steam" to access its subcontents and last but not least, folder called "Apps". At this point you will need to find the actual AC3 folder, and that folder is called "208480".

In short and sweet, here's the simple bit.

Computer > HKEY_LOCAL_MACHINE > SOFTWARE > Wow6432Node > Valve > Steam > Apps > 208480.

For this bit, you will need to add registry that is not already there. Pay attention to your work carefully.

When you click on the "208480" folder, there should be 4 important "REG_DWORD" type files. If they are not there you must add them.

The four files should be:

DXSetup
Punkbuster
UplayLauncher
VCRedist


You should see and notice which ones you are missing, you must add them. To add them (the same rules apply to all 4) you must right-click, press "New" and then "DWORD (32-bit) Value".
Once you have done that, a new registry should be added, you need to edit the name and type in the name of the registry(s) you are missing from the list above.
Once you have entered the name, you need to double-click the created registry and enter the "Value data" in the space provided. The value data for the registry is simply 1. You do not have to worry about the "Base" section as long as it is a "Hexadecimal".

As mentioned before, you must do the same to all the missing registries.

Now that you are done with the registry, you need to exit the program by clicking the red X of the window. There is no save option as the saving bit happens as you do the editing.

When you have exited the program, you simply need to launch Assassin's Creed III from your Steam library without any frustration on the installation! Have fun and happy gaming! =]

If the video doesn't work, you can watch it directly on YouTube here OR make sure you have Adobe installed for "other browsers" so it can suit Steam: http://get.adobe.com/flashplayer/otherversions/

P.S. There are captions, use them if you need to! =)

Guide created by: RedDeadKiller
Sources of information: Forums around the internet
Programs used: Regedit, Steam
OS used: Windows 7 Home Premium (64-bit)

I would like to thank you all for taking your time to read and use this method. As this is my very first guide, I would appreciate your ratings greatly. I would also like to thank all those on the forums around the internet (sorry, forgot your names!) for this information and help, for not only me, but the whole AC3 Steam community. Thank you! =]

P.S. Feel free to contact me about anything you're unsure about within this guide and I'll try to get back to you ASAP.