Depends on the operating system how you do it, but BOINC stores its tasks in a separate data directory, away from its program files.
You can setup the data directory on another drive, no problem.

In Windows, the default data directory is a hidden directory at C:\Programdata\BOINC\
You can move this directory elsewhere (do close BOINC down before you do). Then you can either go the registry route and change things there, or uninstall BOINC, reinstall it and in the third screen in the installer, click Advanced, then set the path to the data directory (second option from the top) to where you just moved your data directory to, then complete the installation.

If you want to go the registry route, the key to change is the DATADIR key in HKEY_LOCAL_MACHINE\SOFTWARE\Space Sciences Laboratory, U.C. Berkeley\BOINC Setup
No need to reboot the computer afterwards.