I just had to go through a lot of playing with configurations to get the DSTA adventure to work without modifying my normal Minecraft install and figured I'd share considering how nice it is to have multiple versions of Minecraft easily switched between.
The main thing you will need is Magic Launcher, it handles installing mods as well as launching the game in a way that makes this all possible.
I'll do this in walkthrough form with explanation of why each step is done the way it is. In this walkthrough we'll be downloading an older version of minecraft and setting up a configuration for it in Magic Launcher to demonstrate the easy switching capabilities.
Creating a template:
1) Press and hold the 'windows key' (looks like a flag on bottom row of your keyboard) and press 'e' to open an explorer window. On the left side click on the drive labeled 'C:' (it may have additional text present).
2) Right-click in an empty area of the 'file view' (right half), on context menu that appear select 'new -> folder', name this folder 'Minecraft'.
3) Download Magic Launcher into the folder you just created, or copy it in if you already downloaded it.
4) Right-click in an empty area of the 'Minecraft' folder and select 'new -> folder', name this one '_template'. This will be a template used to create folders for any additional versions of Minecraft you decide to set up.
5) Go into the '_template' folder and create another folder called 'minecraft' and a folder called 'mods'.
6) Hold down the shift key on your keyboard and right click an empty area of the 'file view', select 'Open command window here'.
7) In the command window type in 'rename minecraft .minecraft' without the quotation marks. Sadly Windows' file explorer does not let us directly create folders that start with a period, so we must rename an existing folder with a command prompt.
8) Open the freshly renamed folder and create another folder called 'bin'.
9) Open another explorer window (step 1 if you forget how), in the address bar type in '%appdata%/.minecraft/bin' and hit enter. As long as you did not move your minecraft folder this should be your current minecraft installation.
10) Hold down 'ctrl' on your keyboard and single-left-click the following folder and files: natives, jinput.jar, lwjgl.jar, lwjgl_util.jar. With those selected and 'ctrl' still held press 'c' on your keyboard.
11) Switch back to the other explorer window (the one opened to 'C:\Minecraft\_template\.minecraft\bin') and click on the empty area in the 'file view', now hold 'ctrl' and press 'v' on your keyboard to paste a copy of those files.
At this point you have a template folder you can copy and rename which has everything required to launch a fresh copy of minecraft except for the minecraft java module (the '.jar' file). Never add anything into this template folder that could break during a minecraft update, that would defeat the purpose of having a template.
Installing a second version of Minecraft and setting up Magic Launcher to run it (using 1.4.6 as an example):
0) Go to 'C:\Minecraft'.
1) Right-click on '_template', hit 'copy', and then right-click on the open area below it and hit 'paste', this should create a folder that reads '_template - Copy'.
2) Right-click the folder that ends with 'Copy' and hit 'rename', type in '1.4.6'.
3) Download Minecraft 1.4.6 into 'C:\Minecraft\1.4.6\.minecraft\bin'.
4) Open Magic Launcher which should be in 'C:\Minecraft' if you've been following along.
5) Click on the 'Setup' button next to the login prompt in the lower right corner.
6) Click on 'New' in the top right corner, type in a name where it says 'Configuration', I'll call mine '1.4.6'. The 'Copy from' box is irrelevant since we'll be changing all of the important settings in the configuration. Press 'OK' when you are done filling the name in.
7) Click on 'Select' in the top right corner, click on the drop down box next to 'Look In:' along the top and select 'C:' from the list, locate and double-click the Minecraft folder we just created. Inside here double-click the '1.4.6' folder, then '.minecraft', then 'bin', and finally 'minecraft.jar'. The left text-box next to the 'Select' button should display the version number (1.4.6) now, and the right text-box the filename (minecraft.jar).
8) Click on 'Advanced' below the text boxes. Feel free to set 'Window Size' settings as you like, as well as the 'Memory' setting, but beware that setting memory too low or too high can lead to the game crashing with 'Out of Memory' errors.
9) Check the 'Custom' checkbox on the 'Base folder' line, and then click on 'Select' button next to it. You can either navigate to the folder yourself or type in the path. The path in this case is 'C:\Minecraft\1.4.6\.minecraft'. This path must end with '.minecraft' which is why we created a template named that way. Simply press the 'Open' button once the 'Folder path' is set accordingly.
(Recommended) Click 'Test' to make sure it starts up properly, I do this whenever I change any settings or add any mods to my configurations, at a minimum go into a flatland world if your just trying to rush through to make sure it works.
10) Press 'OK' to close this screen and return to Magic Launcher's main screen.
At this point the main screen should have two configurations, 'Default' (which you can't remove) and '1.4.6'. The 'Default' configuration should be pointed at your main Minecraft install without any additional setup.
If you would like to load any mods (including ModLoader):
1) Drop the 'zip' files into the 'mods' folder that is outside of the '.minecraft' folder:
2) Go into 'Setup' in magic launcher. Select the 'Configuration' you want to add the mod to.
3) Press 'Add' on the right hand side and navigate to the mod(s) you downloaded into the 'mods' folder.
4) Some mods may need their order adjusted in the list, but most mods will tell you in the mod list if they have any errors or conflicts. Typically if a mod has an error it either needs to be shuffled around or your missing a mod that it depends on. The order is from top-to-bottom the order it will apply the mods to the game upon launch, and it will do so in a way that is not permanent.
You can create multiple configurations per-install to handle multiple mod setups, which is very handy for adventure maps that were designed for a specific version of Minecraft and a specific set of mods.
Just try to keep your various folders for different versions all sorted so that only mods that work in a particular version exist in it's accompanied 'mods' folder (not the mod folder within .minecraft, but the other one we created). This helps tremendously with stopping you from confusing yourself a month or two later when you forget which mods work with which versions.