Why does my AVG Anti-Virus randomly begin hogging 50% of my computer's processing power?

Thats the deal WITH ALL Anti-Virus Software. You cant do anything about it. Personally I don't Anti-Virus Software because of the processing power it hogs up too. Instead I use MBAM,Hijackthis,CCleaner. But because of this i often have to reformat every 3-4 months.