Select Start/Control Panel/Add or Remove Programs. Select BOINC, then click Remove.
Mac OS X
You do not need to uninstall BOINC on the Mac if you are installing a different version. The installer automatically takes care of removing the unwanted files.
To completely remove (uninstall) BOINC from your Macintosh, run the Uninstall BOINC application which is included with the installer.
NOTE: for Mac versions prior to 6.8, this also removes all your BOINC Data files. If you want to preserve these, rename or move the folder /Library/Application Support/BOINC Data and each user's BOINC Manager settings at /Users/[username]/Library/Preferences/BOINC Manager Preferences before running the Uninstall BOINC application.
For Mac version 6.8 and later, the uninstaller does not touch the BOINC Data files. If you wish to delete them, drag the items listed above into the trash after running the Uninstall BOINC application.