Float drop problem in ie 6.0

Hi
I am having a problem getting my pages to display properly in internet explorer 6.0. They are fine in all other browsers.
Dreamweaver is telling me it is a float drop problem.
The main content div is dropping below the bottom of the sidebar div.

An example of the page with the problem is…

[url]http://www.barbicanmums.com/localresources/pregnancybaby/antenatal.html[/url]

Code for my sidebar / container and main content divs is as follows-

.thrColFixHdr #mainContent {
margin-left: 180px;
padding-top: 10px;
padding-right: 10px;
padding-bottom: 10px;
padding-left: 20px;
font-family: Geneva, Arial, Helvetica, sans-serif;
font-size: 12px;
top: 10px;
line-height: 16px;
border-left-width: medium;
border-left-style: solid;
border-left-color: #ea530a;
color: #545454;
margin-right: 10px;
}

.thrColFixHdr #container {
width: 780px;
background: #FFFFFF;
margin: 0 auto;
border: medium solid #ea530a;
text-align: left;
}
.thrColFixHdr #sidebar1 {
float: left;
width: 150px;
background: #ffffff;
font-family: Geneva, Arial, Helvetica, sans-serif;
color: #545454;
padding-top: 0px;
padding-left: 15px;
font-size: 12px;
height: auto;
line-height: normal;
border-right-style: none;
border-top-style: none;
border-bottom-style: none;
border-left-style: none;
padding-right: 15px;
word-spacing: 1px;
}

I am new at this and all help would be VERY VERY MUCH APPRECIATED!!!
Many thanks

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