The answer has always been different, Norton, Kaspersky, Panda, Avast, Microsoft Security Essentials(One Care), and AVG. I have tried alot of them but my fall back has never changed.
Even after running scan with an antivirus, you should always get a second opinion. I've used and recommended this program to anyone I talk to, "Malwarebytes Anti-Malware". The free version is great, and the paid real-time protection is even better.
When your just not sure if there is something on your computer, download this program and run a scan you might just be surpised by what your anti-virus has missed.