From first glance it looked just like SharePoint Designer. I wanted to be sure of that, and tried to install designer on the same machine. guess what? I got an error.
ok- I removed Expression from my machine and then - my sharepoint started throwing script errors at me! "Library not registered" in every page!
After crying and reparing sharepoint and even restoring the entire "12" folder, I realized when the problem started (uninstalling expression) and I knew how to fix it- I installed sharepoint designer!
This time the installation worked, and afterwards sharepoint stopped antagonizing me.