Many months ago I downloaded an add-on for MSN Messenger, I've used this program often, and its quite good, however I didn't notice that this program had added a 'custom install' feature. As it turns out, I didn't use the 'custom install' feature but had I known what the normal install was adding to my system, I would have.
Immediately after install I noted that my browser's homepage was now "lop.com". Do not, ever, go to lop.com or install anything related to it. Not only was my browser homepage hijacked, but my search engines, and my system. I could neither change my browser homepage (it would alway change back to lop.com) nor the search page, nor the crazy pop-ups that began filling the screen. My download had added a browser hijacker, spyware and adware to my system.
After some research I found several great applications which I have begun using in my day-to-day operations at my work. Whenever someone complains about their PC being slow, I install these aps and start hunting. The aps can be found here.
I highly recommend hijackthis, ad-aware and Spybot S&D. There may be other good resources at http://www.spywareinfo.com/, however these are the ones I've used.
Hijackthis is perhaps the most powerful of the three, however, it also is the most technical of them. If you've never messed around in your system's registry, I highly recommend not using Hijackthis.
Finally, here is a bit of news about what browser hijackers can end up doing to your life and reputation.
Remember, if you're not sure when a pop-up window appears... DO NOT CLICK "OK"!