I like Kaspersky but... A free combo I use on many computers. -Avast Free (Anti-virus.) -Secunia PSI 2.0 Free (Audits Windows software for security issues/updates.) -SnoopFree Privay (Monitors keyboard / screen access.) -ZoneAlarm Free (Firewall.) -IO Secutiy 360 (Misc. security) They all work well together, no performance issues that I've observed and give you a good range of protection.
I'm using a free version of AVG and it works well and it detects almost every virus on the PC. This helped and helps me a lot.
Another fan of AVG, I've used most of the popular free programs and that's the one that runs the most efficient for me and isn't too much of a resource hogger (for me at least).
i use mocrosoft security essentials with registry mechanic, both are so far good for me, didn't get any problem. using for more than 6 months..
I use Norton Internet Security, my little daughter also uses it with norton online family instead my son the free version of avast.
At work we use Forefront which is actually pretty awesome and at home I use Security Essentials. It's free and does the job.