The experts' clear choice is Microsoft Security Essentials, nearly invisble after you install, update it, and it runs its first scan.
Second choice would be Avast which is only slightly more visible.
Paid AV's are bloatware infections which even require special tools to remove. Avoid.
Do your MS updates and especially be on the lookout for newer drivers
in Optional updates.
Suggest you install programs slowly with time in between to gauge performance changes. Don't let any write themselves into msconfig>Startup list as they freeload on CPU/RAM and don't need to start until you click them. Only AV and gadgets/stickynotes need to start with Windows 7.
When you get it setup as you want, clean and order HD with CCleaner "Run Cleaner" and Registry tab, Auslogics Disk and Registry Defraggers, then save a Windows 7 Backup image externally so you never have to reinstall again - just reimage the HD or a replacement in 20 minutes.