Horizontal form layout

Currently I'm working on a form with, on a few lines, 2 single line text fields next to each other.

To position the two 2 single line text fields on the same line I'm using the following css:

/*left section*/
#field_2_3, #field_2_4, #field_2_8,#field_2_15,#field_2_17{
float: left;
width: 45%;
clear: both;
}

/*right section*/
#field_2_16,#field_2_18{
float: left;
width: 45%;
}

.button{
float: left;
width: 100px;
margin: 15px 0px 0px 0px;
clear: both;
}

an example can be seen here:

http://dev.villageocelandes.fr/huren

I have the following questions:

1. the css I'm using seems not to work well with IE7. How can this be fixed?
2. How can I expand some of the input fields? Now they all the same size.

Also suggestions for a different css approach are more than welcome. It would be nice to have a video tutorial on how to position 2 or more fields on the same line.

Thanks,
Walt

Thanks for the prompt response. It seems to work fine when I use the css below:

li#field_2_15, li#field_2_17 {width:55%; float:left}
li#field_2_3 {clear:both}

input#input_2_15, input#input_2_17 {width:296px;}
input#input_2_16, input#input_2_18 {width:200px;}

Thanks again,
Walt