A recent update to Apple’s operating system (Snow Leopard) has caused some trouble with Parallels virtual machines. This can be fixed by following these detailed steps in the Parallels Support Knowledge Base.
According to their website:
After updating the Mac OS X to version 10.6.8, with Parallels Desktop 6 for Mac installed, the Mac system’s Dock process may start consuming 100% of the CPU core. This results in slower than expected Mac OS operations.
Parallels KB Article ID: 111541