(This also works on Vista as well). First, to successfully install, you can't use the autorun/menu setup - it will fail. To refresh your memory, this is the menu where you click the button to install Paperport 8. Instead, open the CD up (explore) to folder with the Paperport.msi file. Co-located with that file is the Setup.exe (this is the setup.exe file that is called by the menu program when you click the button in the menu to install Paperport).
Second, Right-Click on the setup.exe (the one co-located with the Paperport.msi file), and select run as administrator. DO NOT try to run the MSI file - it will fail. Just use the Setup.exe file. Once in, answer the questions and complete the install.
The only issue I ran into (in Vista 32/64 and Windows 7 64) was that it can't create the Paperport scanner printer driver. This is the one used when printing - you select the paperport printer driver and whatever you are printing is redirected to an image/doc file in paperport instead of the printer.