PC just stops working?

iam reading my mail 20 `minuetes ago and bingo everything just froze could not even shut down ,had to shut down at plug have run avast antivirus and reg cure,its working at the moment but this has happened twice today.i have no great knowlege of computers as i have only just started useing one about 2 months ago ,so if someone told me to boot something up i would not have any idea what they were on about.