"Input not supported" is a message from your monitor (not from your computer). It means that the monitor does not support the signal that your computer outputs.
This can be because the resolution is too high, or because the refresh rate is set to a value that your monitor does not support.
There are two possible solutions:
- Reinstall the game (cleanly, no leftover config files!). This might work since most games start an a safe setting and you usually need to increase the resolution to one which works well with your hardware.
- Connect it to a display which can handle the setting (e.g. an old CRT monitor, or an LCD monitor which allows for more input values) and then change the resolution of the game to something which your other monitor supports.
Since your almost always want to run in an even multiple of a LCDs native resolution I suggest setting the resolution to that.
Note that you can change the resolution without starting warcraft by editing its settings in the registry. (start, run, regedit. Then browse to HKEY_CURRENT_USER, software, Blizzard entertainment, warcraft III, Video; then change