Why does poclbm acquire 100% CPU? I recently switched from WinXP32bit to Windows7. While under XP poclbm took about 5% CPU Time of one core, now it uses one core with 100%. Is there a specific reason for that? 

I now use AMD/ATI OpenCL1.1 and poclbm with poclbm.exe -device=0 -v -w128 -f 60. MHash/s is about the same I had with XP.
This is a common problem whose cause is not well known. You can try switching miners, switching flags, or switching OpenCL versions. Usually you can find a combination that resolves the problem. I've seen this question asked several times and I've not yet seen a solid answer.