I need to format my system so frequently..please helpp?

i use windows xp and nowadays i started using internet..and within 2-3 months i have formatted my sstem so frequently possibly because of viruses ..two days ago my computer stopped working and i formatted the whole system..after that i downloaded some internet essentials like idm,firefox,opera, and avast antivirus and pcsuite..after two days when i started mw computer it does not work properly..it hanged..nothing worke..and again i formtted my system..please tell me is it a virus attack..??And how can i prevent this problem...