The ideal option is a laptop with WinXP and all the consequences:)
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.
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:)