Google toolbar re-installing itself?

I have had the Google toolbar installed for ages. A little while ago I got a window on my screen saying thanks for installing Google Toolbar. It wanted me to check the box to choose whether or not to show the page rank and then click Finish. It wouldn't let me close out of it. I tried rebooting my computer and it was still there. I tried Control,Alt,Delete and closed out of Firefox but when I tried to open Firefox again it was still there. I did a system restore and it was still there when I opened Firefox. Finally I had to check Disable page rank and click Finish. Then it opened up Google as my home page (DG is my normal home page). It had changed the icons I had on my Google toolbar. The whole process was what I would expect if I were downloading the Google Toolbar for the first time. Does anybody know why this happened?