thanks for your input. I went as far as restarting with the Leopard disk and performed a HD and permissions repair, all successful (HD didnt require repair; quite a lot of permissions were repaired). However, I quit the osx installation, restarted and now I'm back to where it started, back to the gray screen with logo and and spinning circle...
Initially how it all started yesterday was the following: I was scanning about 10 paper documents using a wifi Brother all-in one printer, over my network. Never has any issues with that. I saved the scanned jpegs on my desktop and saved them as a pdf (because I need to mail them), created a new folder and dropped them in it and when I wanted to archive that folder, the cursor turned into the spinning rainbow. After some 30 minutes I turned down the machine holding the power button. Since then, it restarts, the start-up sound goes off and then it gets stuck on the gray screen.
Should I run another osx installation?