Base SDK missing after a fresh install of XCode 3.2.5 which includes iOS SDK 4.2

I'm new to iPhone development. I just did a fresh install of XCode 3.2.5 with iOS SDK 4.2. No previous XCode or iOS versions have been installed before. However trying to compile 开发者_StackOverflow中文版some sample project from some books gives "Base SDK Missing". :( What to do? How can I use XCode 3.2.5 for older iPhones like 3GS if it only supports 4.2?

When I go to "Project/Edit Project Settings/Architecture/Base SDK/iOS SDK's" only iOS 4.2 is available to be selected.

Do I need to install older XCode first and then upgrade?

In the build settings, you can support older phones by setting "iOS Deployment Target" to something like 3.0. http://developer.apple.com/library/mac/#documentation/DeveloperTools/Conceptual/cross_development/Configuring/configuring.html has more details on the difference between this and the Base SDK setting.

Set Project/Edit Project Settings/Architecture/Base SDK to the Latest OS version, then quit and restart Xcode. You can later set the Deployment target to a lower OS version.

I had the same problem for most of the afternoon. I did everything suggested and nothing worked UNTIL I cleaned and cleaned and saved and rebooted and re-built. Unfortunately that doesn't tell me what actually worked, but it does give everyone "just one more thing" to try.