So I found a lot of threads on several forums about this problem but no solution at all.

GUIMiner with rpcminer-cuda.exe (latest version) always freezes my PC for good when I click "Stop mining" in order to... stop mining.

Windows 7 x64
E8400
9800 GTX
4 GiB RAM

It also happens on my ION2 Netbook (EeePC 1215N, Windows 7 x64, 4 GiB).

Latest drivers on both.

Yes, I know. Both systems are completely unsuitable for mining. I don't intend to seriously mine anyway.

Solution:
Turn off behavior monitoring of Microsoft Security Essentials.