Had the same over here and found a solution. You probably have a non US Windows version and the installer is missing the appropriate RevitInstallXXX.dll where the XXX stands for the language code.
Workaround:
1. download webinstaller but don't run it
2. instead unzip the exe and put in a directory.
3. copy your RevitInstallXXX.dll file from where you have installed 5.1 to this directory
4. start installation with RevitInstall.exe
It did work for me ;-)
