So I started Casemodding.
After looking on a couple of mod-sites I desided that I could do with some ccfl lights and a window. Later I came up with the idea of making a little stroboscoop for the CPU-fan.
I took a 555 timer and made this stroboscoop with 5 white LED's and aimed them on my fan
By adjusting the pulsewidth and frequency of the timer I could see my fan as if it was standing still... see the little video on this page
How it works:
The timer makes the led's go on and off with a frequency that is an equivalent
of the fan rotation speed.