Uninstall all versions of IE9 from the system.
Then download and run the appropriate Microsoft SUR tool
Reboot and try installing IE9 again.
Be sure to select the proper version of both of these programs.
I just tried to install IE9 on my W7 64bit system and it would not install, it stalled with no errors, tried 3 times, stalled during download phase and no errors, had to kill it in task manager.
What I did to get it installed.
Performed a clean boot using msconfig, use step 1 and 2 in the article below and restart the pc and install IE9, if IE installs, reboot again, then use msconfig again to revert the changes you made earlier, restart one last time.
Step 1: Start the System Configuration Utility
Click Start, type msconfig in the Start Search box, and then press ENTER.
If you are prompted for an administrator password or for a confirmation, type the password, or provide confirmation.
Step 2: Configure Selective Startup options
In the System Configuration Utility dialog box, click Selective Startup on the General tab.
Click to clear the Load Startup Items check box.
Click the Services tab.
Click to select the Hide All Microsoft Services check box.
Click Disable All, and then click OK.
When you are prompted, click Restart.