Switch to Firefox is right. In the meantime, download PrevX and scan for malware just as a precaution.
Which version of IE? Is the back button missing? Or is it grayed out or you click it and nothing happens? Do you click it and IE changes to 'webpage cannot be displayed' or something like that? Do you have Google friend connect or other Google stuff installed? A few more details, please.
It appears on the toolbar just as it should but when you click on it nothing happens. Same with the Refresh button.
I read a forum on this issue and they said to "disable query suggestions" on Google. I did this and nothing happened. I rebooted and now everything is fine. After reading that forum my guess is that this will reoccur.