Error: ‘isIE’ is undefined

OK Windows XP, Internet Explorer, that’s what I’m running on my pc.
I get an error some times when I use the back button when I’m on the internet.
It says a run time error has occurred.
do you wish to debug?
Error: ‘isIE’ is undifined!
I know is releated to Internet Explorer but what does it means? and must important how do i fix it?
would IE update do?
please be detailed!
Thanks!