All beta builds of Windows 7 will begin shutting down after every two hours of use starting today. Microsoft wants you to start using the Release Candidate build of Windows 7, and this is their loving way of showing it. Not sure what version you’re using? The Release Candidate has build number 7100, so anything before that is considered a beta build.
You can download the 32-bit and 64-bit versions of the Release Candidate build here: http://www.microsoft.com/windows/windows-7/download.aspx