Okay, after I've downloaded the wrong file yesterday (which I've figured out after the 16 hours download), I finally downloaded the right one (read just the Service Pack 1, not Windows Vista including the Service Pack 1) and I've started the installation now:
I've also fired up my stop watch to see if the installation really takes 30 minutes only (which would be fast for a 1.x GB Service Pack) or if it takes longer anyway.
UPDATE: Okay, the installation of the files took something around 25 minutes only. Now the PC rebooted and Widows is configuring the updates (as it said).Just in time while I was reading this update, the PC rebooted again. We will see what's coming next...
UPDATE: Okay, after roughly 25 minutes of installation and another 30 minutes of configuration the PC came up again is is ready to use now: