1. Make a shortcut for your Diablo II.exe
2. Go int Properties (rightclick on the shortcut and at the bottom of the list)
3. There is a line like "C:\Program Files\Diablo II\Diablo II.exe", (it can be different if u installed D2 in another directory)
4. Just add -w at the end of the line, so it looks like this:
"D:\Diablo II\Diablo II.exe" -w
5.Done