There should be c:\Program_something (depending if you have 32 or 64 bit windows), then there sould be /Blender somewhere around.
Under the Blender folder in Program_something there should be 2.75/scripts. This is one place to look if your addon is there (usually some kind of folder, could be a simple python file (ends in .py; you have to enable “Show Extensions…” in windows, if you do not know how, google).
Another place where addons end up is Documents/User/Local Settings/Blender Foundation/2.75/addons, something like this.
Depending on how and what you have installed addon might be in one or in both places. Delete related files, install again normally.
*** To see folder Local Settings you’d need to enable “Show Hidden Files and Folders”; again if you do not know how - google.***
For some of addons you’d need to run Blender as an Administrator because addon might try to write data near the folder it lives. As i get it, if that happens to be Program Files/and_so_on, your rights to write in there can be limited to zilch and addon will miserably fail (usually ending in cryptic error message not telling directly it can’t write data). In folder /User/Local Settings/ user should have more rights - data addon will try to write will be saved.
Hope helps or gives some ideas.