According to the release notes, the latest build of Parallels Desktop 6 for Mac fixed a longstanding problem in which the shared clipboard between a Windows guest OS and the Mac OS would break. When this occurs, it's no longer possible to copy/paste between operating systems. Unfortunately, the bug is not fixed. It happened to me again yesterday. Very frustrating. There is, however, way to temporarily restore the connection between the clipboards. In the VM's configuration, go to Advanced and uncheck the Share Mac Clipboard item, then click OK. Return to the Advanced preferences, recheck it, and click OK. The clipboard sharing should work again. You don't have to power down the VM to do this.