The more free security downloads you install, the more problems you will have.
This is true to a point.
Do NOT download and install any programs based on pop-ups or ads you see while surfing.
There are many legitimate and effective tools that ARE free. A few examples for antivirus; Avast, AVG, Microsoft Security Essentials. For antmalware; Malwarebytes, SuperAntiSpyware, and again Microsoft Security Essentials.
Many of these have paid versions, and for many people that is a good way to go.
Just be sure when you get programs like this, get them from reputable sites that check their downloads for malware e.g. download.com, tucows, majorgeeks.