You mention that the HD has only been installed for 4 days - what does this mean? Was the HD reformatted, or was the OS simply re-installed (i.e. is it possible that files/machine configuration could still exist on the drive/in the registry)?
Depending on the product, WebPI determines whether a product is installed using different methods, such as checking a registry key or a file version number - this check is done before the product list options are shown, so WebPI should not even show you that you can install a product if you already have it on your machine.
Could you please indicate where it was shown that the product was previously installed (i.e. WebPI logs after install failure or somewhere else)? Also, if you could please specify the product you were installing, it would help to troubleshoot why WebPI thought it was already installed, and we may be able to provide you with a straightforward workaround/explanation.