Can’t open or Install Picasa 3

Hi guys,
I was using Picasa for a while on my PC. I used to see all my pictures on it. But now I am getting problems to open the Picasa, not even I can install the program. An error message appeared in the screen like: "Picasa cannot close Picasa. Make sure all users have closed the program”. But there are no other users. All I want to be able to install Picasa 3 and collect my pictures. I need your help.
Thank you in advance.
