Have you downloaded any updates for Parallels 6?
Here's something I found on macwindows.com
Parallels releases update 6.0.11994 for 10.6.5/10.6.6
Monday, February 28, 2011
Parallels has released Parallels Desktop 6.0.11994 for Mac, a maintenance update to the virtualization software for Mac OS X. The release notes indicate that update "resolves compatibility issues " specifically for Mac OS X 10.6.5 and 10.6.6.
If this update fixes any problems for you please let us know. (Related: Parallels Tips and Reports)