BOINC must not be installed as a service (Windows) or a daemon (GNU/Linux) because this project uses Virtual Machines. When BOINC is installed as a service or a daemon it runs as an unprivileged user and cannot run any virtual machine.
The best solution to solve this problem under Windows is to install BOINC disabling the Service option in the Windows installer.
Late news: BOINC Client 7.2.10 and above solve this problem for running as a service under Windows.
For GNU/Linux users the recommendation is to use the official BOINC GNU/Linux installer instead of the packaged ones for your distribution.