php_gd.dll and Windows XP

I've recently upgraded to XP Pro from Windows Me. I previously had PHP running with the gd extension on Omnihttpd server, but found with the same configuration gd would not work (Error messages: First: " The procedure entry point add_assoc_long could not be located in the dll php4ts.dll" Then: "Unable to locate dll 'c:\httpd\php\php_gd.dll' - the specified procedure could not be found". I've also tried installing with Easywindows on IIS - again, gd does not work (though no helpful error messages). So I'm coming to the conclusion there some incompatability between XP and the php_gd.dll. Does anyone have this working on XP? Can anyone suggest how I can fix this? Many thanks Alan Hale