http://forums.iis.net/t/1178550.aspx
If WebPI itself is failing to install - Post the log file here: %temp%\webpi_launcher.txt
If WebPI is installed, but an application or product fails to install: Post the latest log file from here:
Win7/Vista/Win2k8 and above: %localappdata%\microsoft\web platform installer\logs\webpi - grab the latest WebPIx.log file
XP/Win2k3: %userprofile%\local settings\application data\microsoft\web platform installer\logs\webpi - grab the latest WebPIx.log file
WebPI is installed and you know specifically which product is failing: You can look for the installer log file from the component that failed.
Win7/Vista/Win2k8 and above: %localappdata%\microsoft\web platform installer\logs\install\<datetimestamp>
XP/Win2k3: %userprofile%\local settings\application data\microsoft\web platform installer\logs\install\<datetimestamp>