Here is a link to the research topics of World Community Grid, one of the projects BOINC offers. There are detailed explanations of exactly what your computer is doing to help each project, and you can choose exactly what sub-project you want to support (or pick all of them).

For the sub-project mapping cancer markers:

Mapping Cancer Markers on World Community Grid aims to identify the markers associated with various types of cancer. The project is analyzing millions of data points collected from thousands of healthy and cancerous patient tissue samples. These include tissues with lung, ovarian, prostate, pancreatic and breast cancers. By comparing these different data points, researchers aim to identify patterns of markers for different cancers and correlate them with different outcomes, including responsiveness to various treatment options.