Unfortunately, there is no quick fix to that problem. IE 7 is not standards compliant. Microsoft has put in a lot of work to ensure that IE 8 is standards compliant. IE 8 does have a number of modes that will allow it to work the same as IE 7. In IE 8 you can press F12 to open the developer tools window and then have a play around with the Browser Mode and Document Mode settings.
To get your application cross-browser compliant will unfortunately take a fair amount of work. You will find that there are a number of differnece in how the browsers handle html, css and javascript.
Expect it to be a painful process to get your website to be cross-browser compliant especially if it is a fairly sizable website.