Your question may be better posed to one of the forums here.
Windows 7: http://social.answers.microsoft.com/Forums/en-US/category/windows7
However, the answer is that you can choose to upgrade from Vista to Windows 7 or you can perform a clean install of Windows 7.
Before doing either, you'll want to run the compatibility wizard - http://windows.microsoft.com/en-US/windows-vista/Start-the-Program-Compatibility-Wizard
You will need to uninstall OneCare *before* upgrading.
Check the report results for any other programs or devices that the wizard reports will have problems and check with the vendors for updates.
-steve
~ Microsoft MVP Windows Live ~ Windows Live OneCare| Live Mesh|MS Security Essentials Forums Moderator ~