I get that problem on occasion too. Instead of leaving it to sit for 15 minutes, I close Carrara and work on the computer, in about half an hour if I open Carrara it might be back to normal, if not, I restart the computer and then it definitely is back to normal. At least so far rebooting always fixed it. I use XP 64, though.

I also noticed that this phenomenon is more likely to happen when I watch streaming video while Carrara is actively running (doing some computations, or I go back and forth while the streaming is on). But this could just be a coincidence, or it could be the particularity of the problem on my hardware.