Which Outlook is compatible with Windows 7
Microsoft states that only Outlook 2003, 2007, 2010, and 2013 are supported on Windows 7. This also applies to Vista which also doesn’t support earlier versions of Outlook. You can install a version prior to Outlook 2003 on Windows 7 but be warned, there are several problems which you could encounter.
On top of these version requirements, Outlook 2003 and Outlook 2007 still have several compatibility issues which could cause you problems on Windows 7. The most critical issues have been taken care off in prior Service Pack releases. So before starting Outlook on Windows 7, make sure you install Service Pack 3 for Office 2003 and Service Pack 2 for Office 2007. Also note, the 64-bit version of Outlook 2010 and 2013 also requires the 64-bit version of Windows 7.
Outlook 2010 and now 2013 are officially supported by Windows 7.
How about Windows 8?
So far, Microsoft says the exact same thing about Windows 8 as they do about windows 7. One drawback is that currently (even Outlook 2013) no Outlook version is compatible with Live Tile Support on Windows 8. Fear not though, you can still use the Live Tiles of the default Mail, Calendar and People apps and the Lock Screen show information which you have stored in Outlook. For details see: Live Tiles for Outlook on Windows 8