Error message - there is no attribute "onload"

All,

I get the following Error message when validating by document

 1. Error /Line 239, Column 235/: there is no attribute "onload"

    |...HideLayers('drive','','hide')" onload=*"*MM_showHideLayers('drive','','hide')" /...|

    You have used the attribute named above in your document, but the
    document type you are using does not support that attribute for this
    element. This error is often caused by incorrect use of the "Strict"
    document type with a document that uses frames (e.g. you must use
    the "Transitional" document type to get the "target" attribute), or
    by using vendor proprietary extensions such as "marginheight" (this
    is usually fixed by using CSS to achieve the desired effect instead).

    This error may also result if the element itself is not supported in
    the document type you are using, as an undefined element will have
    no supported attributes; in this case, see the element-undefined
    error message for further information.

    How to fix: check the spelling and case of the element and
    attribute, (Remember XHTML is all lower-case) and/or check that they
    are both allowed in the chosen document type, and/or use CSS instead
    of this attribute. If you received this error when using the <embed>
    element to incorporate flash media in a Web page, see the FAQ item
    on valid flash <http://validator.w3.org/docs/help.html#faq-flash>.

Please can you explain why I get this error.  I don 't understand when I 
am using the onload command on several different lines of code and they 
don't produce an error.

Please find attached a zipped version of the  document concerned.


Regards,

Susan Pidcock