Well, I'm not home yet but thanks to an iPad I'm able to post on the train, so here is a how to to have more than one minecraft version on the one computer (also allows for different mods etc)
First go to where ever you want your new minecraft install to live and Create the following directories:
Copy Minecraft.exe to minecraft\bin (this is the file you download from minecraft's website)
Copy your .minecraft directory to minecraft\data
Your .minecraft directory contains these words, and is located in your user profile (c:\Users\yourlogin\AppData\Roaming\ under Vista, or C:\Documents and Settings\yourlogin\Application Data\ under Windows XP). And you have to have played minecraft at least once for it to be there
Then create minecraft.bat file(you can call it anything just don't leave of the .bat), containing the following instructions:
In order to create a bat file you can just create a text file with notepad and then add the text above and then change the file extension from .txt to .bat
Then, simply double-click minecraft.bat to play the game!
Altering the APPDATA environment variable does not touch the system APPDATA. This change will only affect programmes launched from the bat file. It will not change your system settings.
If you are on a Mac I'' still figuring out how to do it, I know it can be done just haven't figured it out, otherwise you could create a seperate user account and just switch users (fairly quick to do, just can't do other stuff from the other user at the same time)