Personally, I use MAMP. It takes only minutes to set up. I also use VMWare Fusion for Windows testing, although I haven't tested whether the MAMP files can be processed that way using a shared folder (I keep meaning to). I'm sure Parallels would be just as good. Anyhow, the only thing I find I need to test on Windows is the CSS and JS, rather than the server-side stuff.
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