I ran into an issue a few weeks ago where I could not get XCode 3.2.6 with iOS 4.3 to install on my OS X Snow Leopard machine. After frustration and research I finally found the solution.
I had an error saying “XCode Install Failed” (can’t remember exact message)
To fix this, try the following in order:
1. Make sure you have installed all updates to 10.6.8 Snow Leopard
If that doesn’t fix it, try
2. Downloading and installing Apple Software Installer Update 1.0
As a last resort, try
3. Re-installing Snow Leopard on your machine and run all updates