Here's what I'd do:
1.) Look in Add/Remove programs and uninstall the previous versions, if installed. The entries should be "Macrommedia Flash Player" and "Macromedia Shockwave Player."
2.) Down a fresh copy of the installers...Shockwave and Flash Player.
3.) Clean up any remnents using the free programs CCleaner and RegCleaner.
4.) Make sure you are logged into an administrative account on Windows.
5.) Exit any unnecessary programs, including your browser(s).
6.) Try the installation again.

Hope this helps,
John