How isolated are Windows and Wubi-Ubuntu installations?

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http://askubuntu.com – If I install Ubuntu via Wubi, I would like to still use Windows for many things, like playing some games not available on Ubuntu. While using Ubuntu, if I encountered an executable that would normally infect Windows, would anything happen to the host Windows operating system? Would Windows have the virus that I downloaded, or are the two operating systems completely separated? Another thing: Are files saved in Windows openable from Ubuntu and files saved in Ubuntu are openable from Windows? (HowTos)