Are you running Windows Vista Beta versions?
If so, you must've already received notifications that your beta/pre-release versions are about to expire!
To avoid any loss of work, please move upgrade to the latest/release version of Windows Vista.
Here's more information:
And if you are too lazy to read the whole thing, here's the excerpt...
On May 31, 2007, 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 notifications about the upcoming expiration on May 18, 2007. To avoid work disruption and the loss of data, 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 prior to May 31.
To install the final version of Windows Vista on any of these pre-release versions of Windows Vista, you may purchase the desired Windows Vista Upgrade. The Windows Vista Upgrade must be the same language version as the pre-release version you are replacing.
If you participated in the Customer Preview Program, you have been using a pre-release version of Windows Vista Ultimate. Windows Vista Ultimate is the most complete edition of Windows Vista—with the power, security, and mobility features that you need for work, and all of the entertainment features that you want for fun.