You are looking at wrong path. webpi.preferences file is located at

C:\Documents and Settings\Administrator\Application Data\Microsoft\Web Platform Installer

Whereas logs will be at:

C:\Documents and Settings\Administrator\Local Settings\Application Data\Microsoft\Web Platform Installer\logs

See the path difference after C:\Documents and Settings\Administrator.

There should be some log. It is impossible not to have those.

Thanks,

Don.