It seems to be pretty powerful and has found lots of stuff on a few of my Windows (9x and 2k) machines. It also checks the registry for lots of nasty stuff that can go on behind your back.
I investigated tools like this because I was having trouble with my dial-on-demand internet connection staying off when I hung up...
[link|http://security.kolla.de/|Spybot Search and Destroy]. Freeware with a $5 donation requested.
It may not help your blocking problem, but might help your mysterious access problems.
Good luck!
[Edit - lost replaced by lots]
Cheers,
Scott.