By doing the following,
body
{
background: url( yourBG ) repeat-x;
}
you should not have any problem... Your background will scroll as intended.
If this wasn't helpful, try updating your question by adding some more code example of what you're doing.
ADDED AFTER POSTING COMMENTS :
HTML :
<body>
<div id="background"></div>
<div id="yourContent">
...
</div>
</body>
CSS :
#background
{
position: relative;
top: 0px;
left: 0px;
width: 100%;
height: 100%;
background: url(images/bg_slice.png) repeat;
z-index: 1;
}
...
I've used that approach more than once, so I am positive it SHOULD work.