I probably should have known about this bug a long time ago, but apparently Windows IE (I’ve only tested v6) likes to double your margins on floating DIVs. What’s the solution? A tiny-ass, stupid line of code:

display: inline;

Other CSS tips:
How To Clear Floats Without Structural Markup – now this one I have known about for a while and its very handy.
What I like to do when I’m testing out my CSS is to turn on borders all all DIVs in my document. Many times it takes the guessing out of why your page is behaving like it is:

div {
border: 1px #333 solid;
}

and if by some chance you’re old school and having table issues, you can do the same:

table td {
border: 1px #333 solid;
}