Windows 7 doesn't come with an AntiVirus pre-installed. Malwarebytes is NOT an AntiVirus. An infected USB drive is all that will be required to infect your PC. Please consider a proper AntiVirus like Kaspersky, Avast, ESET. Also Malwarebytes doesn't come with a firewall. Your post states you have a third party firewall but you have not specified what you use. And I agree with the rest. Use a good backup software (even the inbuilt will suffice).