Hey shadowvzs
You can also try installing sandboxie when you want to run several instances of warcraft 3 at the same time.
Basically this:
http://www.youtube.com/watch?v=qmjt3NWNYV4
Can be used for any instance of an application. Not only for Diablo II but also Warcraft 3
(Don't forget that you'll need several cd keys if you want to test it on battle.net!,
To do this install warcraft 3 multiple times on different locations of your HD's or SSD's)
Another option would be:
creating several windows accounts, then turn off UAC and setting up a fast switch for switching between 2 windows accounts. (need 2 installations on HD's or SSD's)
Another option would be:
run the warcraft 3 instance within a virtual machine like
https://www.virtualbox.org/ (1 installation on HD or SSD and 1 on the virtual machine)
Another option would be:
getting a desktop switcher like
http://vdm.codeplex.com/
(yet again 2 installations needed, don't know if this one will work though haven't tested it yet)
All the other options should work, it's just a matter of prefference.
I gave you 4 options ^.^ pick 1.
My personal favorite is the virtual machine ofcourse
Right now I got 4 operating systems which I can use in my computer due to virtual machines: Windows, Backtrack, OpenSuse, CentOS