Microsoft Office 2007 Windows Installation Error 1500

"Another installation is in progress. You must complete that installation before continuing this one."

When the Windows Installer performs a transaction, it creates a registry key called InProgress, which is located at HKEY_Local_Machine\Software\Microsoft\Windows\Installer\InProgress. When the transaction is done, it removes the key. Other installations check for this key before they begin. If the key exists, the installation does not start and produces Error 1500. To clear the problem you can run MSIzap with the P switch, "msizap.exe p". MSIZap is provided in the Windows Installer SDK and also comes with (the much smaller) installation of the Windows Installer Cleanup Utility. This utility itself is worth getting your hands on- it provides a nice GUI interface to forcably removing software. If you are stuck installing a vendor-provided installation and run into this, try rebooting your system in safe mode (press F8 during startup to choose). This is a known and corrected problem in MS Office!!!!!!!!!!!!!!!!!