I can't analyze exactly what's happening in your site just from the snippet you posted above, but I can tell you that IE6 is notorious for having many annoying quirks and bugs with CSS rendering. It's one of the things that makes web design a real pain in the *** -- that you get everything right and then have to test for that piece of ****.
Anyway, without knowing your exact problem, I can see that you have a float and margin in the same direction of the float. This triggers a bug in IE6 where the margin is doubled. There are some fixes for this. Please see
or google "ie6 float margin css" and variations of that.
PS Your code above looks a bit screwy also. For starters you have the float declared twice. Have you run this through a validator?