what to install on a computer running Windows 7 proffesional?
XnView or XnView MP?
Why not install both and see which you prefer?
XnView 'Classic' is mature, but XnViewMP which is in beta and still lacks some features is developing well, has an attractive new interface and also supports Unicode characters if you need that...
Note: If you wish to open PDF files you need to also install Ghostscript, x32 for XnView 'Classic' and x32 or x64 for XnViewMP depending on which version you install.
- List of missing features (compared to XnView Classic)