What kind of virus is this....? another folder and sending automatic link tru yahoo messenger?

what kind of virus is this all of my file have duplicate folder on it. and automatically sending a link from my yahoo messenger group list. i already format my C: but how can i remove that by not formating my D: i have allot of important files there. ^_^ im using avast inti virus it can black the trojan virus but if i check my folder theres a duplicate folder again. and if i restart my PC will pop avast to black the virus again and again. also all of our computer here at home infected already it is very fast. need help from the expert.... ^_^ thanks..... ^_^