Okay. Like a lot of people on here, I just recently tried to start Fallout 2 for the first time. I continually got the message that Fallout 2 was running in compatiblity mode. I have read several posts from other users having the same problem, and other personsposting how to solve it, and more still asking for step by step instructions on how to do it. So, I'm going to try to explain what I did. Feel free to reply.
First, I went to http://store.steampowered.com
and downloaded the Fallout 2 Restoration Project (version 2.2), Windows Installer.DO NOT RUN IT YET.
If you do, when you get to the part about where to install it and try to install it where Fallout 2 is located in your Steamapps, you will get a pop-up saying that it cannot change your program files without possibly damaging your computer.
Instead, what you are going to do next is locate the Fallout 2 folder in steamapps on your hard drive. It should be located someplace like this:
C:\Program Files (x86)\Steam\steamapps\common.
It may not be drive C on your computer, that will depend on where drive is your hard drive(and which hard drive you have steam installed on in the event you have multiple hard drives on your computer.
Now, you are going to right click the folder that says Fallout 2 and select cut. Next, go to your drives and right click on the icon for your hard drive and select paste. This should move your Fallout 2 folder to the first level of of your hard drive (i.e. you should be able to see the folder on the first list of folders that appear when you double click your hard drive's icon).
Next, you are going to run the Fallout 2 Restoration Project (version 2.2), Windows Installer, which should be located in your downloads. When you get to the screen that asks you to select your destination folder, you can either click browse and find the folder in your hard drive. If you have selected it correctly, it should look something like this in the box:
Just finish running the installer and wait for all of the files to download/extract. After it has finished, go to your harddrive, right click on your Fallout 2 folder and select cut, again.
Finally, go back to C:\Program Files (x86)\Steam\steamapps and right click the folder that says common, and select paste. This should put it back into steam and et you launch the game from Steam as you normally would. If you have any questions, feel free to post. I apologize if this sounds too patronizing, but I wanted to make this as simple as possible for persons who are not adept at computers. For those who are curious, I am using Windows 8. Most of this should be even simpler if you are using Windows 7.