CS4 software has not been tested and is incompatible with Windows 7.
If it was working fine for you before, you can uninstall the software completely by using the Cleaner tool ( http://www.adobe.com/support/contact/cscleanertool.html )
And then download and install the software from http://prodesigntools.com/download-adobe-cs4-and-cs3-free-trials-here.html
Note : Make sure you follow the important steps mentioned on the page before initiating the download.
Since the problem started after installing the Nik software plugin. Have you tried uninstalling it.
Further you can try disabling start up items ( Ref. http://helpx.adobe.com/x-productkb/global/disable-startup-items-services-windows.html )
Also try enabling the hidden admin account on Windows 7 ( http://www.howtogeek.com/howto/windows-vista/enable-the-hidden-administrator-account-on-wi ndows-vista/ ) and try installing on this account and check
Unfortuneately your latest suggestions didn't help either.
I normally have an image of the installation as backup, but this seems to be corrupt for some reason. Can I really have the situation that I have to make a clean installation of my systems disk to overcome the installation problem? This would take days.