Check out the float tutorial.
IE has problems with the float model (although apparently there's improvement in IE 8). You could try using just absolute positioning... even though that's probably not the ideal solution.
Thanks all, Katesmeow probably helped the most:
Turns out what I needed to do was absolutely position the main div and the left (menu) div (which I gave a minimum height) and then relatively position the content div. I made the left menu 18% wide with a 1% left margin and the content div I made 80% wide with a 1% right margin. I also had to add bottom padding to the main div according to the top offset that I gave to the content div (so I used "top: 30px" to give the main div padding from my content div and then used "padding-bottom: 60px" to compensate 30px for the top - because otherwise the div extends too far - and then add a further 30px of padding).
The problem is that IE still mucks it up; so while my main div is 95% wide, IE doesn't do the relative widths properly and my main content pane is too wide. So, I used conditional comments and set the content div to be 76% wide (95% x 80% = 76%) and it's working reasonably well. The rounded corners are doing something slightly odd in IE but it's fine... The biggest problem now is that I don't have a bottom margin on the main div but I can live with that if I can't figure it out.