As a web developer, older versions of IE can be a little tricky to work with. They require a little more time and attention than Chrome or Firefox. HTML and CSS rules are somehow different in IE than in other browsers so it’s important to make sure your site works in IE. The absolute best way would be to download a VM image of Windows XP and actually run IE6 and IE7 to test your site. But, if you’re not into installing a VM of XP, there’s something you can do: IETester.
I’ve used it a couple times and it seems to have a few bugs in it, but overall its a nice free tool to check what your site looks like in IE.