Javascript getelementby tagname problem!

im doing a clientside gridview calculation

in javascripti have used

var gv = document.getElementById("<%= GridView1.ClientID %>");
var tb = gv.getElementsByTagName("input");
var lb = gv.getElementsByTagName("span");

and in the gridview i have a textbox, the calculation goes perfect but when i add select all checkbox the calculation comes right but total gets populated in alternate column.

when i remove the select all checkboxs it words fine but if i add the checkbox the total gets populated in alternate column.