I.E v Opera error

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I admin a couple sites, i designed the pages in bog standard html, when scripting them i was using I.E, the pages were fine all worked lovely,
I then started to use Opera, one of my sites layouts is a little out of shape, like i have messed up the table commands or something, but IE shows it as fine,
Its weird,

I spose it doesnt really make too much bother, i've just designed a flash layout instead so im getting rid of this [I]problem[/] layout,
That and about, well to be exact 67.86% of the sites viewers are using I.E , but i still like perfection :-))