We also go to all kinds of interesting lengths to avoid problems with viruses and worms. For example, we have a hack in our flavor of Wine, in the CreateProcess call (the code to start an executable) that basically checks to see if the parent process is outlook.exe, and if it is, we crash and burn, preventing many of the worms and such from running.
Meanwhile, someone's porting WINE to MacOS X. It doesn't actually emulate an Intel CPU, so it won't run your Windows binaries, but you can recompile Windows programs from source code and get them to run, and look authentically Windowslike, on your Mac. Though you'll need to use X11 as well, as it doesn't speak directly to Quartz/Cocoa/Carbon (and there don't appear to be any plans to make it do so).
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