Someone else's downloads!!!

Hello all! I have kind of bad problem. Well, for me at least. My fiancee acquired a list showing things I supposedly downloaded and she's questioning some of the stuff now. A number of the files she's asking me about I know for a fact I didn't download, but apparently this list (which came from monitoring software) is showing my username and IP address were used. These are somehow showing as received files on my computer. Does anyone have any ideas as to how this could happen? I would really appreciate any help anyone can offer. Thanks a lot!