AVG. But I've moved to free Avast as it gets better reviews. I also run CCleaner, using it about once a month to clean out the "junk". In addition to these I have MalwareBytes and Super Anti-spyware. I run these two about once a week.
The caution you need to take is whether these are active scanners. If so they can conflict. Avast is active and always running in the background. The other two are manual start (free) and do not interfere with each other.
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