from Microsoft, Malacious Software tool from Microsoft, turn on automatic update and you should do fine. All from microsoft.com. Don't have to buy anything more. Once a week I will run the disk cleaner then do a full scan from Security Essentials.
Once a month, I would do a scan disk for disk failure. Than do a disk defragmentation. Go to Windows update website and check for hardware device updates. The automatic updates don't update devices.
I only run Lavasoft Ad-Aware and Spybot only as needed.
Had several PCs humming for years without problems.
