You need to change your skin1/customer/main/product.tpl template.
Find this code:
Code:
{if $product.fulldescr ne ""}{$product.fulldescr}{else}{$product.descr}{/if}
And change it to this code:
Code:
{if $product.fulldescr ne ""}{eval var=$product.fulldescr}{else}{eval var=$product.descr}{/if}
This is untested, but should work for you.