I'm using poclbm to mine under Windows... processor is an AMD A6-3400M with integrated Radeon HD 6250G GPU. When Windows Media Player is closed, I get a rate of 16 megahashes/second, but when it is open (even if stopped or paused) poclbm reports a rate of around 33 megahashes/second.
I'm wondering if this has something to do with Windows Media Player increasing the system timer resolution, but that doesn't seem like it would affect performance.
Ideally I would like to achieve maximum GPU performance without having Media Player constantly open.