As the resident "IT Whiz" at work (ie: I know how to use a computer) I've been asked to develop a website for our small business. I've altered a wordpress theme for the time being and the company is very happy with the results. The only problem I am having with it at the moment is that for some reason the website does not display correctly and Internet Explorer unless I run it in Compatibility Mode.

The main problem that I have is that my menu "slider" (it rotates pictures with links to articles etc) does not display at all, neither does the top menu they are just blink text based links.

Even with Compatibility Mode enabled the slider and menus come back but the page is not centered unlike on both Firefox and Chrome. My googling has suggested the most common cause of this is old code but I'm not sure where to be looking. Is it likely in the css file or the actual php?

Also any ideas on how to trouble shoot the cause of this? As in is there some dev tools or debugger I can use that would highlight "broken" code for me?