Please see this thread, I think it will have the answers :)

http://www.worldcommunitygrid.org/forums/wcg/viewthread_thread,35605

On quick scan, it does not seem the 'pramo' referred thread contains the answer [albeit it gives info on other ways to reduce chance of loss]. Logging off and loosing progress [probably BOINC unloading completely], means that BOINC was probably installed at user level. Would you uninstall and install again, then taking the 'advanced' installer option to run BOINC as service [if you have the rights of admin], the client would continue, even when logged out. When doing that, make sure to also select 'allow all users' in that advanced installer screen [the second one after the 'I agree' software ladihdah.

Alternately, instead of logging out, you could use the Windows Key+L to lock the session. This would even allow for a different user to sign in, after which you could hot-swap sessions without loosing progress, and LAIM on of course. [notebook ... assumes that's running a form of MS OS, but that is not always the case ;]

edit: In additions to the 'windows' assumption, 'logging off' is understood as -not- powering down.