look at your registary, something has been put there to do this, hackers tend to choose easy targets like yahoo, face book, twitter etc to hack, if you cant spot it in your registary then its easy to clean the hard-drive and reinstall windows, back up everything you want to save
Once done ,you are advised to get back online to windows and update your windows wether its xp or vista etc, you will be suprized then how quick the computer has become after this, make sure that you have the correct disc to do this.
