There are a few options out there for free antivirus software. What I would recommend is AVG.
The free version of AVG can do all of the following:
- Blocks viruses, spyware, & other malware
- Scans web, Twitter, & Facebook links
- Securely deletes files to prevent snooping
- Full System scan
- Quick System scan
- Scheduled Scans
- RootKit Scans
As a piece of free software, it is very capable, and I've yet to see any viruses arise in computers that use it, but that's not to say it is perfect. AVG does offer a paid version for USD $54.99 that has additional features such as:
- Download protection
- File encryption
- Online shopping protection
Whether these features are worth the charge is up to you, however the features in the free version are more than capable of protecting you from everyday viruses.