The ideal option is a laptop with WinXP and all the consequences:)
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The most reliable compromise — a clean WinXP in virtual reality. Virtual box is able to integrate seamlessly, just like a window. However, you need to utility at least 512MB memory virtual reality to be at least minimally comfortable.
On Macs there are wonderful Parallels.
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Everything else is just not able to reproduce a broad range of glitches, bugs and crashes inherent only in IE6, in all its pristine glory:)