Why does merely having Windows Media Player open double my GPU hashrate? 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.
poclbm has not been set to alter the clock rate of your GPU. Windows media player is doing this when open and adjusts the system to what it requires.

If you specify clock speeds for your GPU via parameters or a separate application you can overcome this. 

Theres a number of applications which effect clock rates on windows machines, flash player open within a browser reduces a single card of mine down from 400 m/hash to 350 m/hash.

cgminer is very easy to specify clock speeds and mine with, it adjusts them when you start the application (not when closing though).