It's been supposed that if IE9 was installed before updating to SP1, then it can't be uninstalled without first uninstalling SP1, but if the SP1 backup files were removed, then SP1 cannot be uninstalled as well. Do you have SP1 installed? Do you know whether IE9 was installed before SP1? Did you ever run Disk Cleanup
and remove Service Pack Backup Files
This may explain why you cannot revert to IE8.
After all of this, did you try your game in IE9 in compatibility mode? Internet Explorer Compatibility View - Turn On or Off