/<x-cart_dir>/skin1/customer/main/products.tpl
Code:
{if $products[product].discount ne 0}
<font class=MarketPrice>{$lng.lbl_market_price}: <s>{math equation="(price/(100-discount))*100" price=$products[product].price discount=$products[product].discount format="%d.00" assign=unformatted}{include file="currency.tpl" value=$unformatted}</s></font>
{/if}
Remove the <s> and </s>