Looking at the page http://purebredcats.org/cat_health.htm ...
Quite a few problems there, the biggest may be the lack of DocType. Have a look at what the validator finds - http://validator.w3.org/check?verbos...cat_health.htm
See the links about validation in my signature line. There are also links about DocTypes and Tables there.
You have .menu set at width: 250px; but the content you put in it is much wider than that. The box model shows how content cannot be larger than it's containing element - margin/padding/border all count when figuring sizes.
Try commenting out that width and see how it looks.
border-top: 2px solid #9197AC;
border-bottom: 2px solid #9197AC;