Sorry for not being very precise in the topic, but i couldn't come up with anything more fitting.
I was looking at my workstation while sitting in front of the tv and was noticing that new graphics ran in the BOINC screensaver.
So i went over to check it out and i noticed some weird glitches in the graphics. They seemed to switch between two frames, one of them appearing correctly, the other one being distorted.
So i moved my mouse to exit the screensaver and my secondary monitor turned back to my desktop, but my primary monitor didn't. It still showed the screensaver, although without any animation.
I tried shutting down all BOINC related tasks, but that didn't help.
In the end, all i could do was a reboot.
I disabled the screensaver for now, so i can't say if it still happens. Running the graphics manually seems to work well.

I'm running Windows XPSP2 on a P4 640 with a Radeon X770 installed.