hasn't happened to me here yet. it's likely you have spyware, first sympton is usually slower system performance, and web sites being hijacked and redirected to 'other' unwanted sites like ones selling stuff, and bad sites you can't talk about here...
- also that suggestion about using firefox stopping pop-ups is a good call. I use it- just keep it updated...
if you have time go to
http://www.bleepingcomputer.com/download/anti-virus/combofix - small application- just expect it to run about an hour...
this is a dos-like application that will update your pc, scan it and remove most (if not almost all!!) spyware and junk/scumware. it even gets the hard ones removed with out formatting and reloading your pc.
best way I've used it- download the software to your PC, restart your PC -press F8 -choose safe mode with networking and run the application. if your are connected on a RJ54 cable you should be in good shape, if you have wireless it might not update due to no internet in safe mode. running the application it should update, take a while to run, and end up with a notepad msg saying it removed what ever it found. (usually a lot of junk).
you should notice a good difference in the computer web surfing.
hope that helps some if you decide to check it out.