First I would go to run, type msconfig and hit enter, go to the startup tab and if it shows up in there disable it then restart. Then check addons for firefox/chrome and see if it's in there. Does it show up in task manager (probably under some obscure name)? I can't remember if you can do this in XP or vista, but in windows 7 and up you can right click on a process and select "Go to file location", if you end the process you should be able to just straight up delete the executable for it.