Solution
I had the same problem. For some reason I lost mouse synchronization. If I clicked inside OS window, I would lose mouse and keyboard alltogether, both having no affect on eithe host or guest OS, until I pressed Ctl+Alt to free them. I could then click on USB icon in Parallels, and choose to connect internal Apple Trackpad and keyboard. In device manager of the guest OS (XP), I had standard HID keyboard and mouse, and also Parallels devices, which showed exclamation points in yellow circles.
To fix the issue, I disabled conflicting devices, uninstalled Parallels tools. Reboot. Then install the tools back. Go in XP device manager, enable Parallels mouse and keyboard. Reboot. Voila! That fixed it for me.
Good luck!