it is the name of the links image being named "links". It would not surprise me if that name is reserved for something else in the browser.
I saved the page and renamed "links" to "link2" just to test and it worked then:
Code:
<a href="links.html" onMouseOut="MM_swapImgRestore()" onMouseOver="MM_swapImage('links2','','images/nav/links-over.gif',1)"><img src="images/nav/links.gif" name="links2" width="74" height="19" border="0"></a>