Button appearance changes when given set width

I have a default asp:Button control with no styling set. It renders nicely in IE7 with rounded edges etc. as the stadard button should.

However when i set a specific width to the button, i.e. Width="200px", the button is rendered differently in the browser, with jagged edges and a slightly different look and colour to it.

Does anyone know how to get around this so that all my buttons look the same (the nice standard look!), even when i give them a set width?

Hope this makes some sort of sense...

Thanks
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