file association execution

hi,
i am new to microsoft operating system. always likes unix more :)
anyway, i was trying to use the registry to locate the branch which holds the executable for a specific file type so i could add parameters to it.

suppose, when i double click *.docx file, i would like winword to load the file and use the parameter /n, as follows for each *.docx:
winword filename.docx /n

where is the location on the registry which i can modify each file association executable?

thanks.

acrobat for windows opens one instance per a file. i.e. when double clicking firstFile.pdf an acrobat instance will start and it will display the pdf file. when minimizing the instance and having double click again on the same file, firstFile.pdf, the previous (minimized) instance will get to the foreground and will display the file; whereas i wanted to have two separate instances of acrobat.

analyzing acrobat parameters, acrobat can get the parameter of /n for executing a new instance of acrobat.

reading your post, i knew i am looking at the right place :)
so i went to hkey_classes_root\acrobat\shell\open\command then i've edited the string value to include the /n parameter. but it did not had the desire effect i was looking. (and i did restart my machine).

any ideas how it should be done? why does the /n parameter is being "ignored"?