Ehh.. this only supports Windows, first of all. Mac and Linux users get nothing out of this. Fine, you did say so in the topic. But I'd definitely want to support those platforms as well.
I'd be happy to explain the code too:
Button1.Enabled = False
My.Computer.Network.DownloadFile("Put your dropbox link here", "C:\Program Files\.minecraft\mods")
MsgBox("Mod Installed!", 0, "Auto Installer")
Button1.Enabled = True
Buttons are, in Visual Basic .NET, by default named ButtonN, where N is the number of buttons after the button is placed. Hence the first button you place is called Button1. The first line simply tells that button to disable itself - Java coders; this is the equivalent of jButton1.setEnabled(false).
The second line tells the program to download a file from a given link and save it C:\Program Files\.minecraft\mods. I can already sense trouble here. First of all, Minecraft's installation directory is not in Program Files. It is in %appdata%. Even getting the AppData folder path almost requires more code than is present overall here. What this essentially means is that if you haven't created a junction shortcut (symlink) from %appdata%\.minecraft to C:\Program Files\.minecraft\mods, the installer won't work. And most people don't even know what a symlink or junction shortcut even is. To add on to this, it will be saved as a file named "mods" in the .minecraft folder, it will not be installed inside the mods folder.
The third line displays a message box saying "Mod Installed!", with just an OK button, and the title "Auto Installer". Java developers may be familiar with JOptionPane for displaying message boxes like these. If you don't know what I mean by message boxes - if you're using Windows, open Notepad, enter MsgBox "Hello world" in there, then save the file as a .vbs file and run it. Those are the kind of boxes I refer to.
The fourth line is the opposite of the first line, enabling the install button. There isn't really even point in doing this, because the user isn't going to install the mod over again.
As for the fifth and final line, I'm not even sure if its syntactically correct. The proper word to use would be End. And even End is very brute force. It's like shutting down the app the same way as through the task manager, by the End Process method. The best way to shut down an application would be through Application.Exit().
To summarize all this: The code posted by the OP does, to be honest, just not work. Downloading the entire Visual Studio pack, which is well over a gigabyte, just to make an installer is overkill. The output file is an .exe as well, which further reduces the trustworthiness of whoever uploads it. And as if this wasn't enough, applications made with VS 2010 require the .NET Framework 4.0. This is pre-installed on Windows 7 and Windows 8, but anything lower than that requires whoever is using your applications to install it manually. This often takes up to 30 minutes, maybe more. Yes, this means that everyone installing your mod who are using Vista or below have to spend 30+ minutes of their time installing something to even get your installer to run. Furthermore, even if this for some reason would work, it would only install Forge/LiteLoader mods. That's because it simply would've dropped the mod in mods (which the installer doesn't even do right now), and wouldn't have installed things in the Minecraft jar. Installing things there even requires a third party zip library, like DotNetZip.
There are existing, cross-platform solutions to install mods which actually work to some extent. For instance, check out the Mod Installer Creator I made. You don't even have to code anything - you just upload the mod file to a web page, enter the required information and download the installer, ready to use on any platform, as a jar file. It's still buggy, but it works.
I AM NOT YOUR PERSONAL MINECRAFT MOD SUPPORT AGENT, SO PLEASE DO NOT PM ME ABOUT PROBLEMATIC MODS THAT ARE NOT MINE. If you're having trouble/crashes with a mod, you'll have better luck resolving it in this forum section than PMing me. If you already made a topic, be patient about responses. If you have troubles with anything non-Minecraft related on your PC, I might be able to help, though, but no promises. Even though I could wish to be, I'm not a wizard.