If you want to install the GAX/GAT July 2007 CTP, you have to uninstall the February 2007 CTP first. If you have troubles like me, these are the steps :

1. Uninstall all Guidance Packages
2. Uninstall Guidance Automation Toolkit
3. Uninstall Guidance Automation eXtensions. 

Uninstalling GAX gives me the trouble.  At the end I got a blank/empty dialog box with only a "OK" button on it.

To solve this follow the next steps

1. Check if all Guidance Packages are uninstalled
2. Run the CMD prompt in the directory where you're msi is located
3. Run "msiexec /x GuidanceAutomationExtensions.msi"
4. If that isn't working reinstall the GAX using "msiexec /fa GuidanceAutomationExtensions.msi"
5. Run again "msiexec /x GuidanceAutomationExtensions.msi"

This works for me.  (Txs to Greg Cool for these tips).