I found this and did the "Method 2: Test the CD or DVD drive in a 'clean boot' state":http://support.microsoft.com/kb/321641/
The cd-rom worked (Do'h!)
So I figured I had to do some intense research about which program was interfering with the cd-rom.
I restarted in Normal state and tried to access the cdrom again expecting to see Windows Explorer not beeing able to show the content and no error messages, like before.Now a message from "Acer eLock Managment" pops up from the system tray telling med the optical drive is locked!!!
This has never been displayed before nor is this software ever been used on this computer. I started this program and it displays that everything is unlocked(!) and to access the setting one has to create(!) a password (first time use!)!!!
I just closed it without creating a password and tested access to the cdrom again... after all the program told me it was unlocked -> no access + message that it is locked!
Well... I then started the Acer eDataSecurity Management software (also part of the Acer Empowering Technology software-bundle) It has also never been used before so it displayed a licence agreement to accept and asked me to create a password... I just closed all the windows without creating a password.
Now I'm about to stop more testing for today, but I try to access the cd-rom one last time and guess WHAT! It WORKS!!!
Now I just have to test that removable drive but I guess the problem is solved...
So why did the system prevent me from accessing the cd-rom and removable drives and why is it working now? I guess I will never know