If you are running the Visual Studio 2010 and .NET Framework 4.0 CTP under Virtual PC or Hyper-V, you should know that the copy of Visual Studio installed in the VPC is set to expire.
After January 1, 2009, you will no longer be able to launch the Visual Studio in the VS2010 CTP. This is due to a hard-coded expiration date in this CTP. CTP's are pre-release software, so they aren't designed to run forever.
If you haven’t disabled time synchronization before Jan 1 and you start Visual Studio – you will need to re-download the image and disable time synchronization before starting fresh.
Workaround for Virtual PC Users
See Brian Keller’s post on Dealing with Activation Messages for the full details. But essentially it boils down disabling time synchronization under Virtual PC 2007 before the timebomb is hit.
Workaround for Hyper-V Users
If you converted the VPC to run under Windows 2008 Hyper-V using my previous instructions, disabling time synchronization is different. See Cameron Skinner’s post on disabling time synchronization in Hyper-V for the full details. You need to take a snapshot of the VM, shut it down and go into Settings >> Integration Services >> Time Synchronization and disabled it.