First of all, if you're only investing in Gridcoin you can skip this guide in favour of immediately installing the Gridcoin Wallet.
Switch to the "Investor guide". You do not need to install or run BOINC in order to use GRC as a cryptocurrency.
Navigate to the BOINC website, download and install the appropriate BOINC client on each of the computers you wish to crunch BOINC work upon. If you're not crunching projects which require VirtualBox you don't need to install BOINC+VirtualBox.BOINC client downloads