XP vs Win7
Windows 7 is a smooth operating system, if you're a punter end-uzer. I have to run Windows XP for quite a few reasons, least of all for gaming. Civilization 4 WILL NOT WORK in Windows 7. Period. Granted, last patch for it was Sep07, but WTF? Yes, it technically is not Windows' fault, but the software companies (who had PLENTY of advance notice), however, older software/games that were developed and reached end-of-life long before Vista was born have no chance. And of course, VMs don't (yet) have native gfx support, so Civ4 in a VM is out of the window(s) :P
Another thought, what will happen to Windows XP Mode in Win7 when XP hits EoL? Technically, it's a feature of Win7, which should still be supported..... But then, Win8 will probably be out by 2014 (hopefully) and all our problems will go away (again)...