There is a bug in the Joomla 1.5 template called 'JA_Purity', which causes an error like this:
Fatal error: Call to a member function getItems() on a non-object in /***/templates/ja_purity/ja_menus/Base.class.php on line 32.
To fix this error, open the file
templates/ja_purity/ja_menus/Base.class.php, and replace
$menu = @JMenu :: getInstance();
on line 32 with
$menu = &JSite::getMenu();
And your template should be fixed. Cheers.