Error: Object expected

Jeg har et problem, min menu virker kun på nogle computere.
Den brokker sig og siger error: object expected
Her er et af de links der er i min menu:

<a href="content/education.htm" target="iframe"
onmouseover="document.images['button_education'].src='img/button_education_over.jpg'"
onmouseout="document.images['button_education'].src='img/button_education.jpg'"
onmousedown="document.images['button_education'].src='img/button_education.jpg'"
onmouseup="changeImages('button_education', 'img/button_education_over.jpg'); return true;">
<img name="button_education" src="img/button_education.jpg" border="0"></a>