If it is a 'standard' screen saver, syscokids advice will work.
Many of the more involved screen savers have additional files that are required to run them.
After you find the .scr file, search the folder on date and time. Take a close look atany files with the same date and time created as the .scr file.
A right click, and review of the properties should tell you if the files are related.
A few screen savers only install a 'basic' file in the Windows or Windows\System folder.
The file references a program installed in another folder.
Once you find the name of the screen saver, you might want to search for it on the web.
Some screen savers are copyrighted programs that 'technically' require a separate license for each PC they are installed on.
Chas