Consider updating the version of Windows targeted from XP to Windows 7

Currently we target Windows XP which is deprecated and unsupported by Microsoft. Most people are using at least Windows 7(which is also near its end of support by Microsoft, but is still used by some people who object to Microsoft's current directions.). It makes sense to stop supporting Windows XP. Updating this would resolve an issue with modernizing the code base where TCHAR on XP is a local multibyte encoding, but on Windows 7 it should be UTF-16.