If you want to mine on your GPU at the same time as playing games or doing other things, simply use m0mchil's poclbm miner (you can download GUIMiner, which includes poclbm) and add "-f 120" to the command line.

The -f switch tells the miner that you want it to back off so that you can maintain a certain frame rate in other programs, like Everquest 2, while it mines with your video card.

For the CPU miner, just lower the priority of the process using task manager, and it'll only use "spare" CPU resources.