怎么用css定位图层阿？？

用层的时候，分辨率不同的时候往往层的位置不一样，比如说800×600的时候两个图层一个在标题栏一个在中间，但换1024×768时两个图层就都挤到标题栏去了！
哪位大侠帮忙解决一下这个问题！！
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不太明白楼主想要什么样的效果～

给个始终居中的层吧！

1<div id="jnkcLayer" style="position:absolute; left:expression((document.body.clientWidth-this.offsetWidth)/2); top:expression((document.body.clientHeight-this.offsetHeight)/2); width:200px; height:200px; z-index:1; background-color: #009966">始终居中的层</div>

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具体说一下，窗口固定位置图层

1<div id="KBStatic" style="position:absolute; left:3px; top:0px; width:553px; height:70px; z-index:25">
2<font size="5">我是大龙</font></div>

 1<script language="JavaScript">   
 2function KB_keepItInIE(theName,theWantTop,theWantLeft) {   
 3theRealTop=parseInt(document.body.scrollTop)   
 4theTrueTop=theWantTop+theRealTop   
 5document.all[theName].style.top=theTrueTop   
 6theRealLeft=parseInt(document.body.scrollLeft)   
 7theTrueLeft=theWantLeft+theRealLeft   
 8document.all[theName].style.left=theTrueLeft   
 9}   
10function KB_keepItInNN(theName,theWantX,theWantY) {   
11theRealLay=document.layers[theName]   
12theBadX=self.pageYOffset   
13theBadY=self.pageXOffset   
14theRealX=theBadX+theWantX   
15theRealY=theBadY+theWantY   
16theRealLay.moveTo(theRealY,theRealX)   
17}   
18IE4=(document.all)?1:0   
19NN4=(document.layers)?1:0   
20if (IE4)   
21setInterval('KB_keepItInIE("KBStatic",0,0)',1)   
22if (NN4)   
23setInterval('KB_keepItInNN("KBStatic",0,0)',1)   
24</script>