Windows 7: Windows Update error 80246008 : system infected !?

Some days ago out of a sudden my system doesn't work any more smoothly. The only thing I did were some deinstallations of programs and the installation of some downloaded programs - I did this under an account without elevated rights, though I was also prompted to give the administrator password which I regularly did.

Here are the symptoms of my corrupted system :
1. If I boot the system and sign on an Installer process automatically starts. It refers to the Nitro PDF Profesional program ( which I also tried to install some days ago ). The dialog window disappears after some time without further notice.
2. Every installation of a program fails : I get a message box which says that the dirctory C:\ProgramData\Microsoft\Windows could not be created.
3. Now the worst : the Windows Update ends with the error message 80246008. I found that the BIST-service ( Background Intelligent Transfer Service ) can be found but isn't started what obviously hampers the update. The manual start fails.
The problem seems to be known since Vista times.
4. An application which I regularly use and which uses Adobe AIR fails to run out of a sudden. I could add screen captures of the messages, but they are in german language and so probably not helpful here.
5. I found a program ( recently installed ) which causes on startup the Installer to become active trying to install a program which is already installed on the system.
A .msi file out of the folder C:\Program Files\Common Files\Wise Installation Wizard
is activated and requests the administrator password. If I deny this, an infinite loop of Installer-starts begins.



Is this system infected or destroyed by malware ? A complete scan for viruses and Malicious Software gave no hits. I also deactivated some programs on system startup but this had no effect.

May someone have left a nice "present" on my system ?
I have the bad feeling that some uninstall program messed the system - this has disappeared from the system, so it can't be found any more.

Is it possible that the simple visit of an infected Web Page can be so harmful ?