Whether you can install IE8 on your computer depends on whether your IE9 was preinstalled when you bought your computer as an update, or whether it is a native IE9.
If it was installed as an update to IE8, use the following procedure:
Click the Start button , type Programs and Features in the search box, and then click View installed updates in the left pane.
Under Uninstall an update, scroll down to the Microsoft Windows section.
Right-click Windows Internet Explorer 9, click Uninstall, and then, when prompted, click Yes.
Then click one of the following:
Restart now (to finish the process of uninstalling Internet Explorer 9, and restore the previous version of Internet Explorer).
Restart later (to wait until you shut down or restart your computer).
You computer should then revert back to IE8 automatically after you reboot.
However, if IE9 was not installed as an update to IE8, you can only make it work temporarily in IE8 mode by pressing F12 in IE9 to bring up the developer tools. and selecting IE8 under Browser Mode. Try the first method first. Then you will know how your IE9 was installed when your computer was purchased.