Add Avira Antivir to the list of good n free Antivirus programs.
These free ones do however come with limited features. For instance some don't have anti-spam/anti-spyware features or a built-in firewall etc.
So you may have to find free apps that fill-in the gaps as well.
Glary Utilities is a freeware with registry and disk cleaning, privacy protection, performance accelerator and amazing multifunctional tools. It can fix dogged registry errors, wipe off clutters, optimize internet speed, safeguard confidential files and maintain maximum performance.
You can download from http://www.glarysoft.com.