To make it work perfectly on IE6, instead of changing 25% to 24% you can:
Comment out padding:
Code:
#order_steps {       /* Is needed for the centering in IE */
    /*padding: 0 3px;*/
    margin: 10px 10px;
    text-align: center;
     border: 1px solid #000;
     height:70px;
}
Also, to make the display look neat on IE6, you have to make these changes as well:
Code:
.progressbar_active {
/*     background:transparent url(../images/orderstep.png) repeat-x;    */
     background-color: #3DA200; /* Determines the active color of the completed part of the line */
     width:25%;
     float:left;
     height: 5px;
     line-height:5px;
     color: #3DA200;
}

.progressbar_inactive {
    background: url(../images/orderstep.png) repeat-x;    
     background-color: #9A9A9A; /* Determines the active color of the completed part of the line */
     width:25%;
     float:left;
     height: 5px;
    line-height:5px;
     color: #9A9A9A;
}
Take note how I commented out the background img, and added line height.

You have to applied the changes above to all the 4 css files.