Okay, just one more time...
I now use an external powersupply ( like the ones used with laptop-computers ) and got the most silent harddrive I could find and now when I turn on the computer I hear: nothing at all.
It is absolutely quiet, first time I thought it wasn't running but when I turned on the screen Windows was booting allright. My computer is now as serene as my favourite screensaver.
I'm happy
( my wife's laptop is actually noisier, let me see what I can do about that
)