I have installed avast v.7 home edition in my system. Whenever I scan using this, the system shuts down. Does anybody know why and how to solve?
Welcome to the forums, sands59. 
Are you getting any error messages? If so, give the exact message.
Also, tell us about other programs you may have including OS, security, etc.
“Shut down” - as power off, freeze, or just reset?