You can also use a div based layout and give the div an absolute width in pixels.
A related method (usable some day in the near future) is the min-width: rule for an element, set with CSS. Browser support is spotty today, but eventually it *should* ensure that small windows don't completely collapse a layout.
Coupled with max-width: you can declare a liquid layout that doesn't get too wide or too narrow.
It is also possible to create a transparent 1 pixel gif. Then place the gif in the table and give it size variables of width=640 and height=1 this will let you use percentages in the table but never allow the table to shrink smaller then 640. Simple cross platform fix. Also the same gif can be used over and over to give this effect where ever it is required.