Sir,
It may be malware, spyware, etc..... First install the best antispyware software and throughly scan and delete all the spyware. Also update the OS regularly, Use google chrome browser( best i ever), Delete Temp cookies regularly(try ace utilities or cleanup), update antivirus regularly(kaspersky is best, buy full version - total internet protection), Use computer single handly (if your friends is using, create guest account) follow all these steps definetly u have the difference.
Thanks Good Luck
Appreciate to rate as "Fixya"