BOINC Manager 5.10.28 runs distributed computing apps on all 4 cores of my Mac Pro continuously at low priority. If nothing else is running, they will take up to 100% of each core, but don't usually interfere with other apps. There was no problem running Fusion 1.1RC WinXP vm with 2 CPUs active on TIger with BOINC. However with Leopard, the VM grinds to a virtual halt if BOINC is running. Changing the VM to 1 CPU doesn't help. Turning BOINC off restores full VM speed. BOINC doesn't interfere with a Parallels VM in Leopard. It's as if Fusion is running at a lower priority than BOINC under Leopard
Have you talked to the BOINC people to see why this is happening? Perhaps there is a command line switch that can make it better behaved.
I'm posting a report there too. I use the GUI BOINC running as a daemon, not the command line version. I haven't seen other reported problems with BOINC under Leopard.