If you previously downloaded the Beta 2 VPC for Visual Studio 2008, the OS used for that instance has an expiration of November 1, 2007. (Note that this only affects the VPCs, not the setup packages, so if you installed VS2008 on your own OS partition, you're fine.)
Jeff Beehler has a detailed post on the timeout and various workarounds.
However, assuming you move any of your custom data off the VPC instance, it might be easiest to just download the reissued Beta 2 VPCs as described by Jeff in another post.
- Visual Studio Team System 2008 Beta 2 Team Suite VPC
- Visual Studio Team System 2008 Beta 2 Team Suite & Team Foundation Server VPC
- Don't forget you also need the base image (as the above are differencing disks.)
As always, the install packages and VPC images are available from the Visual Studio 2008 Downloads page.