Windows 7 is not officially supported for most newer games because it was already shut down on January 2020 (in other words, it no longer receieved an important updates). Even Windows 8.1. did the same thing a week earlier. The only way to enjoy the best experience for most games is upgrade your OS to Windows 11. Make sure you have a computer that supports 64-Bit CPU, as this OS only provides 64-Bit version.
Even though Blizzard used the updated system requirements from January 2020, at that time Reforged had gone gold, I assume that Blizzard had patched numerous DLL files by the time they released a new update to make it only compatible with Windows 10 and Windows 11.
Also, make sure you have a good computer. A Quad Core CPU with at least 16 GB of RAM and 2 or 4 GB of Video Card RAM prove sufficient for playing the most recent games. Even Reforged struggled a lot when you play on moderate computer, especially with HD graphics being enabled.
If you still want Warcraft III: The Frozen Throne 20 years since the first release and don't mind of getting the new features in Reforged, all you have to do is follow deepstrasz's suggestion. Use the link that he provided, then make sure you installed the game correctly. After that, you must start the game using the "Warcraft III.exe" and NOT the Launcher (or even directly from Battle.net app) as it will install the 1.36 version of Reforged.
My comp will not do Reforged but should handle 1.29 update