You can choose whatever project you want to run on your CPU. It doesn't have to be a Project of The Month. Here's a good listing of all BOINC projects and what their aims are: http://www.overclock.net/t/737931/info-list-of-boinc-projects/0_20. Choose one (or more!) that you like and make sure to join team Overclock.net on them. To maximize your impact on the project, choose one that doesn't support GPUs
If still in doubt, run World Community Grid!