The below given powershell script will clear all items in a list. You should use powershell IDE to run this script.

 

[System.Reflection.Assembly]::Load("Microsoft.SharePoint, Version=12.0.0.0, Culture=neutral, PublicKeyToken=71e9bce111e9429c")

$siteObj = new-object Microsoft.SharePoint.SPSite ( "http://yousiteurl" )
$SPweb = $siteObj.OpenWeb()
"Web is : " + $SPweb.Title

$oList = $SPweb.Lists["you list name"];

$oList.Title + " : Total Items Count is " + $oList.ItemCount

$collListItems = $oList.Items;
$count = $collListItems.Count - 1

for($intIndex = $count; $intIndex -gt -1; $intIndex--)
{
        "Deleting : " + $intIndex
        $collListItems.Delete($intIndex);
}