That's more of a process priority issue than anything else.
My Win XP x64 box runs at 100% constantly, and I haven't had any problems with the system being non-responsive, as long as the GUI/Basic system has a higher priority than boinc.
I never seen a PC windows running a good responsive GUI, not in my house, not in university, not on any pc shop, not on any Friend PC.....Maybe your definition of responsiveness is not the same as mine and you settle for little