(post rewritten)

Here's what I did to get it to work:

- Run GPU Caps Viewer to confirm that there is no OpenCL on my system at the moment.
- Use Driver Sweeper to kill my old nVIDIA drivers (no idea if it would have worked without this step)
- Install ATI Stream SDK 2.2
- Reboot
- Verify in GPU Caps Viewer that OpenCL 1.1 is now installed.

Importantly, ATI Stream SDK 2.1 / OpenCL 1.0 did not work, with a very similar error message. Thanks to m0mchil who helped me out over PM! Getting 150M hash on my HD5770, which is pretty cool if the number isn't lying. The OS remains perfectly responsive.

LobsterMan, if you could, please mention in your steps that Radeon owners should install SDK version no lower than 2.2.