Some users who upgraded to Mathematica 9 for Windows have reported encountering blue-screen system crashes. The system crashes seem to be caused by a Windows bug in handling how we update fonts, and can potentially happen even when not running Mathematica (for example, opening the Fonts control panel might cause a blue screen). The issue only affects Windows machines which had older versions of Mathematica installed.
The issue is rare and was not found during extensive internal and external prerelease testing. However, we're taking the issue very seriously, and we recommend that you don't install Mathematica 9 for Windows until we can provide you with a patched installer, which should be available for download from http://user.wolfram.com in the next few days.
If you have already installed Mathematica 9 for Windows and have not encountered this issue there is nothing you need to do.
If you have installed Mathematica 9 for Windows and have encountered the issue, you can find a solution for the problem here: