Free antivirus programs use the same deffinition updates as their paid counterparts... the differences from paid to free are insignificant.. things like updating; the paid versions can be programmed to update at the time of day you want, free versions update when you start the computer and again in intervals you can program.... hours, days, etc. Paid versions include a firewall, free versions do not... You have to register free versions once a year, you only have to registre the paid versions when you acquire the license and again when the licensed period ends (if you want to renew the subscription).
Free antivirus never crash for being free, as I said, they use the same update deffinitions as their paid versions. Any antivirus can crash if it can't trap a virus for lacking that virus deffinitions, and that can happen with any free or paid version if the virus in question is so new that they don't have the deffinitions for it, or if you neglect to update it.
I have used free Avast for some 8 years and never had an important infection with it... the only severe infection was with AVG in 2006 but I managed to remove the infection manually.
Free vs. Fee: Free and Paid Antivirus Programs Compared