I have 4.72 installed on my nt machine and my registry entry for it looks like this:
$NETSCAPE472 = EXISTKEY("HKEY_LOCAL_MACHINE\SOFTWARE\NETSCAPE\NETSCAPE NAVIGATOR\4.72 (EN)"). So if they haven't changed their registry entry you should be able to use:
$NETSCAPE475 = EXISTKEY("HKEY_LOCAL_MACHINE\SOFTWARE\NETSCAPE\NETSCAPE NAVIGATOR\4.75 (EN)")
If exist <> 0
'Install new version of netscape'
Else
Endif