If anything can be installed on your machine without explicit permission from you, then it means exactly one thing... YOU do not have your properly secured.

Proper security has been viable since XP SP2 - or you could upgrade to a current OS such as Vista (remember XP was scheduled to go out of support last year, but microsoft extended it). Windows 7 goes even further on providing security.

Forget about the "legitimate" peices of software that like to install themselve, if Flash can do it, then so can any type of malware. It would not be suprising to find that every piece of information on your machine has been uploaded to the internet.