I have been trying to solve this problem for days! You could try the Microsoft fix it application but it did not work for me.
I found this solution on Software Tops & Tricks site - it hasn't got rid of it completely but I don't have to keep clicking cancel - it just pops up and goes off again in a couple of seconds - not perfect but until I can find something else it will have to do.
"First click start... then press Run. Type in MSconfig Click on the services tab at the top. And highlight 1 of the services. Then press the 'w' key until you see Windows installer. Uncheck the box to the left of Windows installer, then press apply in the bottom right-hand corner. Now you can close the Microsoft configuration box, but another box should pop up asking you to restart. Click restart now. When your computer turns back on it should be free of the problem. If it is not then I would contact Microsoft about the problem"