Certain directories and programs are protected by the system to that they can only be written to by users with a certain level of permission (e.g. Administrators).
It's possible that your program did not write to such a directory (e.g. C:\Program Files) or invoke such a program (e.g. regedit, to modify your registry).
It's also possible - if you are on an Active Directory environment - your IT department has allowed certain programs to install or allowed you to install programs in certain circumstances - either intentionally or by mistake. The policies set by your IT department are pushed out to your machine invisibly, so you wouldn't be aware anything had changed.
Your best bet is to contact your IT department and notify them. Not only will they likely be able to explain it, but if you've found a security risk they can likely identify and fix it.