if you got a template from Joomlart.com - try
http://www.joomlart.com/joomla/templates/ja-elastica
for example, it's free, as is JA Purity II which is also used by very many Joomla 2.5 installations - and select the Mega Menu (normally the standard option), there is a jQuery error if you browse to the DTRegister user control panel (or any other page from the DTRegister component).
Therefore, the Mega Menu stops working on those pages (and other JS functions).
The Joomlart.com templates are well coded and work fine with almost any other extensions (jQuery noconflict mode).
Can you please look into the issue?