Installation of linux plugins usually works just the way you tried.
This time though, you have tried to run a windows program in linux. This is not generally possible or a good idea.
MeadCo's Neptune is a Netscape-compatible plug-in which hosts Microsoft's WebBrowser control. It will work when embedded in documents rendered by Netscape 3.0, 4.x and 6.x, Opera 4.0 - 5.x and Internet Explorer 4.0 - 6.0 running on any Windows platform with Microsoft's underlying web componentry integrated. Windows 95 / Windows NT 4.0 with Internet Explorer 4.0 core install is the minimum requirement.
Neither of those requirements are fulfilled on a pure linux platform. I dont know if Linspire includes CrossOver or something similar, which would allow that program to run. Anyway, the tool offered by pcpitstop would still not work as it is designed to work around some more frequent windows problems. Luckily there is no need for this on Linux. Frankly, what Neptune does is bring the single largest security nightmare of Internet Explorer's to Netscape. Not a good thing. It is very cool that we dont have or need this kind of software on linux.
It was an executable file so I double clicked and ran from terminal
"an executable"? do you mean a .exe-file? This would be a windows-executable. Those don't work on linux. When you would choose to run this from terminal, it would only produce an error. The only thing you would see in this case is a windows popping up and disappearing before you even really see it. This is probably what happened to you.
You don't need the software offered by PCpitstop.com, which is a very windows focused site.
See the top-most sentence on their site is: Is your PC acting sluggish? Are strange windows inexplicably popping up on your screen? Do you have to reboot your computer because of errors and lockups? Our free computer checkup and diagnostics can help you detect and fix many common PC problems! Our full PC tune-up should be your first stop.
Those are frequent windows problems. Linux doesnt have any of those problems, luckily. That's whats so cool about it! No need for this kind of tools, only a sleek system!
Welcome to the first bastion of a better world