I have the exact same problem and I went even further to reinstall the Stream SDK but yeah nothing works GUIMiner Vanishes and cgminer just crashes when I try to start it.
HD 7870
Driver: 12.8
Stream SDK 2.8?
Windows 8 64Bit
Had a BSOD before that happend
GPU Caps Viewer also crashes... I hope not that I have to reinstall my Windows...
So I'm not the only one. I've abandoned it now and I'm using cgminer with a batch file to fire it up when I start. Works great, so far.