You decide that when you install BOINC on your computer. In order to change the drive, you have to uninstall BOINC and then reinstall telling it to put the data directory in another location. The good news is if you want to keep all of your settings and such, you can copy the BOINC data directory before uninstalling and then copy it to the new location after re-installing. Just make sure you shut down BOINC before copying it.

My suggestion is that you create a new folder on the D drive and call it BOINC.

Within that folder create another called BOINCDATA

Unhide all files and folders

Stop BOINC

Copy the contents of your current C:\program data\BOINC folder to the BOINCDATA folder

Uninstall BOINC and carry on as Rob shows above, making sure you select advanced and tell the installer that the program is to be installed on D:\BOINC and the data on D:\BOINC\BOINCDATA

This just puts everything in one place ...easy to find and work on for the forgetful like me.

If you so choose you can now reverse the process of unhiding files and folders as for BOINC at least you will not need to see those again.

This is perhaps much the same as the suggestions above but puts the whole operation on a separate drive

If you're fooling-around with the registry, you'll also need to manually set the access-rights on the new directory + sub-directories meaning it's much better to use the BOINC-installer.