In my win 7 when I try to install avg antispyware thenn all on a sydden the computer turns off. Help me it's been sufferinjg me for long.

You may be installing the antispyware too late. The system may already be infected. Try to install it in Safe Mode with Networking.

More likely the AVG program is incompatible with Win 7. And since Win 7 has not been released yet, you probably have a Beta version which increases the chance of incompatibility.