what could be the problem?

i am a lady who owns a vauxhall tigra, m reg. it just started overheating upto red, engine sounds fine and moves fine untill it starts overheating and smoke, comes out the bonet,i have tried k seal and radweld and now it seems to be overflowing,meaning instead of leaking radiotor water comes out through the cap and i can hear bubbles too. wat could be the problem .help

doc3
November 2009