First, follow this tutorial
to disable IE8 in Windows 7.
Second, download a malware program such as Malwarebytes' Anti-Malware and clean your computer. You may want to download Ad-Aware as well and run that.
Third, Reinstall/Re-enable IE.
If you don't want to go through that process, just use tabs and open new windows in a new tab that way instead of hitting the back button you can just close the tab.