What is the best free anti-virus software I can download? How to protect the individual folder for my computer and usb driver?

Not knowing what operating system you are running, I'll throw out Avira AntiVir.

You need 3 programs to defend your computer
[1] Anti virus to keep the viruses out
Security News from the net: Latest list of Anti Virus programs
[2] Firewall to keep the bad guys out
Security News from the net: Latest list of Firewalls
[3] Anti Spyware to keep the malware ,pop-up ,password stealer out
Security News from the net: Latest list of Anti Spyware removal tools