Hello

I hope you'll understand something
As the title said, ive a coding problem.
Im currently trying to create my first webby and so, the index page.
So im making a simple table
with a 100% height (it's the problem) with a simple image, a background image for the whole page and another one for the table.
When the code is done, all works.
Now im trying to validate it for xhtml, so i put the correct doctype in the code, and when im seeking at the page, the height has moved to 100 px (the logo height).
I cant have the correct height for the table and i really dont know where's the problem....ive tried to use css to change it but always the same problem...
For information, i use :
Coding : dreamweaver mx & Notepad ++
Navigator : mozilla firefox
If anyone can help me, it'll be really great for me ^^
Thanks
ps: a pic which resume what ive said :
linked pic