There should not be any problem to use Lookout 5.1 on Windows 95. However, I think National Instruments obsolete support for Windows 95, therefore you should consider to upgrade your OS as well (asap).
I highly recommend to use at least Windows 2000 or XP (or XP Home Edition) to build up a SCADA/HMI system.
The Lookout 5.1 installer installes per default into an new NI\Lookout 5.1 folder. Don't forget to copy over all the .ini files you have in your Lookout 4.5.1 folder to keep the settings/behavior.
Hope this helps
Roland