I know I'm late to the party here but I have a solution to this problem if anyone needs it.
Go to a computer with express already installed and copy the express folder,ExprBuilderRes.dll, and ExprBuilderUi.dll from C:\Program Files\Land Desktop 3\ or where ever you have it installed.
Put it on the windows 7 machine in the same location
Then type "config" in autocad and add a file to the support file search path. Browse to inside the express folder you just copied over. Move it to the top
Next type "menuload" and load in the express menu
Then type appload and click on the little suitcase in the lower right corner. Browse to the express folder and add everything in there. It will take a few goes because acad only lets you add in 50 at a time.