a lot of players saying that they can not install Warcraft 1.23 patch due to the Registry error loading key 'Warcraft III\InstallPath' Error. This error occurs when you don't have original copy of Warcraft or you don't have registry entries created for Warcraft 3. You can make your registry entries manually by using Windows Registry Editor!.
How to make Warcraft 3 Registry Entries:
1. Open up regedit (start > run > regedit)
2. Go to HKEY_LOCAL_MACHINE\SOFTWARE\Blizzard Entertainment\
3. Create new key, name it 'Warcraft III' (without quotes)
4. On right side of new key entry, right click and create new string value
5. Name it 'InstallPath' (without quotes)
6. Change the value of InstallPath to wherever you installed it (example: 'c:\warcraft 3\')
7. Reinstall Warcraft 1.23 Patch. (this time your war3 will be installed without any errors)
8. Enjoy! your Warcraft is patched to 1.23.
Subject: How to fix Warcraft 3 Registry Error 
Thu Aug 06, 2009 1:32 am
