I have used it for a couple of years, in XP, and before that in 9x, it has never caused any problems and just (when Set Up) runs at start up for a few seconds and then finishes. It stops other programs from placing BHO's in IE. At the initial set up after first installing it will show you any BHO's that are in IE and you decide whether to keep them or not.
It comes with a very detailed Help program and is well worth a read.