jQuery is giving error in IE browser but working in Firefox.

jQuery is giving error in IE browser but working in Firefox.

Hi,
 
 we have inlcuded follwing js files into our code and working using them.
 
jquery-1.2.6.js
jquery-ui-personalized-1.5.3.js
jquerypngfix.js
 
 
we suddenly getting the js error at the following line.
var $window = jQuery(window);
 
it was working fine earlier. suddenly what happend we dont know. it is showing "object doesnot support this method" in the IE browser. It is working perfectly in Firefox.
 
can anyone help me out.
 
thanks,
Bhavani