OK, I had this same exact problem and became a bit discouraged when I realized no one had ever responded to your query with a solution, so I did a little more digging in different forums. Based on some reccomendations from several different posts, here's what I ended up doing, and it finally worked! (I run 64-bit Windows 7 by the way.)
Open the control panel and then click on "uninstall a program". Then (in this order) I scrolled down to Apple Software Update. Right-click and select "Repair". Then I scrolled down to iTunes, right-click and select "Repair". I then ran the software update again, and it worked like a charm!
I hope this helps others who were having the same problem!