You can try this as the doctype:
<!DOCTYPE html>
That puts IE into standards mode, and is much shorter and easier to remember (and future-proof!)
The validation error sounds like you are outputting some content before the doctype, which is not allowed (not even spaces). Assuming you are using a custom theme, try copy-pasting this code for the doctype function:
function doctype()
{
$this->output( '<!DOCTYPE html>' );
}