The validator is your best tool, HOWEVER I want to warn you. If this is a corporate type environment where IE is the mandated standard everyone needs to use, don't spend too much time on it.
IE is notorious for not following standards so you may spend some time fixing in to run in Firefox only to find out that you've messed it up in IE. It takes alot of time and patience to get it write, ESPECIALLY if your going all CSS and not tables, etc. Once you get it right its worth it, unless your bosses see it as a waste of time and see a reduction in your productivity.
thanks a lot, the validator is really very helpful. As for images not getting displayed the problem was the way the image path specified in code. After changing '\' with '/' the iamge path, it was working fine.