I'm not sure how you could have installed Particle Fire without installing it yourself, but if you ever figure it out I'd be curious to know.
Anyway, all of our installers include an uninstaller that you can use either from Add or Remove Programs, or from the LDA Games/Particle Fire entry in your Start menu.
If those aren't there, as you say, I'd set my watch and warrant that there's a 'pfire.scr' file either in C:\Windows\system32, or C:\Program Files\LDA Games\Particle Fire. If you delete that, it should remove it from your list of screen savers.
If all you want to do is turn off the screen saver, that's pretty easy. Just right-click on an empty part of your desktop wallpaper, choose 'Properties' in the popup window, then on the 'Screen Saver' tab of the window that pops up you can select '(None)' as your screen saver.
Long says and pleasant nights.