Can you install it or just not run it? Because if you can install it, but windows won't let you run it, it may be as simple as running it in compatibility mode. however if that is not the case, and you cannot install it period, you may want to consider making a battle.net account, this account will allow you to add most blizzard games (Sc, diablo 2, warcraft III, WoW, Sc2) to your account via cd key. Once added a game can be downloaded from blizzard and installed. I personally have every blizzard game I own attached to my account.
Here is the site... http://us.battle.net/en/
As for Dota, just go to the site (below) and download/save to your warcraft 3 maps folder.http://www.getdota.com/
Hope this helps =D