On a recent project I needed to be able to detect if a user was browsing using IE8 or 7, and serve different functionality to them if that was true.

After looking around a really easy way to do this is to simply check whether the browsers supports the leadingWhiteSpace feature, using jQuery's $.supports method.

if ($.support.leadingWhitespace == false) {
     var oldIE = true;
}

Old versions of Internet Explorer don't support this feature, and therefore oldIE will be false on these browsers. You could then apply a conditional class to your body tag to style something differently, or write a quick if statement to check the value of oldIE before executing some script for example.