Why would my laptop suddenly start telling me that my copy of Windows Vista is not genuine?

The last time I fired up my (HP) laptop I got a prompt to activate Windows. I had no idea what it was so I closed the dialog box and moved on. When the computer was done booting up there was a message in the bottom right corner saying my copy of Windows Vista is not genuine. This is the same copy that I have run since I bought the PC – pre-loaded of course with the operating system. I can access the web and use my Microsoft programs. Any idea what gives?