I've had a couple of computers crunching part time for the past couple of weeks during the beta testing (2 computers 12 h/day) and like most other distributed computing projects, it's pretty unobtrusive. Download and install the BOINC client and follow the instructions. With E@H, you can tell it the start and end times you want the client to crunch away. When the client detects any user activity it stops, giving you back the CPU until you're finished. Then it goes back to work.
And if gravity waves aren't your thing, well then check out the other BOINC projects