The two best Free antivirus programs are Avast4 click here
AVG antivirus click here
ONLY USE 1 ANTIVIRUS PROGRAM
you will need to register them to use (no cost) but updates are free. Avast free license lasts 14 months then has to be registered again (just done mine again)
See if you have an entry in Control panel - Add/Remove programs to uninstall Stop sign.