Windows Vista Just registered !

Hi every one just got in and already I have a question ......I`m running a HP dual core a1740n one of their desktops with a lot of bells and whistles Premium edition .......It has been running well since Febrary last year with the odd error here and there ( nice machine ......I`m 67 yrs old ) 2 days ago, I do not know why ,a window about 5 inches square appears on the desktop and it happens about 1/2 an hour, or less sometimes after I crank it up,Appreciate any tips thanks guys ...... and the date and time only is on the frame the inside of the window is white .......How do I get it off the desktop ,it`s annoying ..