Antivirus

Avast

A great, real time antivirus protection that find the intruders. Both free and paid

AVG

Free and paid antivirus. It doesn't have real time protection, but it works well

Norton Antivirus

Well known and good antivirus that is included in many pcs when you buy them

Trend Micro

Paid antivirus. Their HouseCall is a very good, free online virus scan

Panda Security

Popular, paid antivirus. It also has a free online virus scan you can use