If you are using Windows Vista Beta 2, RC1 and RC2, prepared for upgrade as set to expire. According to WindowsVista blog:
On 31 May 2007, all Customer Preview Program (CPP) pre-release versions of Windows Vista will expire. If you are running a pre-release version of Windows Vista (Beta 2, RC1 or RC2), you will begin to receive warning notifications about the upcoming expiration on 18 May. To avoid work disruption and loss of data, we strongly recommend that customers running any of these pre-release versions of Windows Vista migrate their PCs to the final version of Windows Vista prior to 31 May.
Now the scary part:
After 31 May 2007, customers who continue to use pre-release versions of Windows Vista will be able to log in for 2-hour sessions to retrieve data. After 2 hours of use, the PC will automatically reboot without providing the opportunity to save data. The opportunity to log in normally for these 2-hour sessions will only be available until 28 August 2007. It is strongly recommended that PC users running any of these pre-release versions of Windows Vista migrate their PCs to the final version of Windows Vista or another operating system prior to 31 May.
So you must upgrade if you like Windows Vista or if you just use PC for Internet browsing and office work you can try out new Ubuntu Linux ;)