To get more detailed info about installation failure do the following.
You extract the Windows installer file PowerPointAddInSetup.msi from the Speech-Over exe file (it is really a zip file) with WinZip and put it in a folder say C:\SpeechOver\PowerPointAddInSetup.msi
Now do installation by command line,
To enter the command ine first do Start/Run/Cmd and then enter:
msiexec /i "C:\SpeechOver\PowerPointAddInSetup.msi" /L*V "C:\temp\SpeechOverInstall.log"
where C:\SpeechOver is the folder where you extracted the installer zip to, and contains the installer file PowerPointAddInSetup.msi. The log is written to C:\temp in this example
The log should contain details about the failure.
For more info about producing a log file of the installation see http://www.advancedinstaller.com/user-guide/qa-log.html
Joel