First things first, make sure you have Diablo 2 installed.
- Right click the Diablo 2 icon
- Left click Properties
- Click Compatibility
- Check the checkbox “Run this program in compatibility for” and select: Windows XP (Service Pack 3)
- Check run in 256 colors
- Check Disable Desktop Composition
- Check Run this Program as Administrator (this will by-pass any User Account Control issues)
- Click Ok
- Now open Diablo 2 (it should now run)
Note: It has also been reported that setting up and adding your Diablo 2 keys to Battle.net will allow you to download a client from Blizzard that will run fine.