Hi deependra

You might find from a little research on other forums that there were problems in the past with vista versions using AVG security software i.e. anti-virus
I can't say if they have all been resolved but it might be worthwhile switching it off or even uninstalling it to see if vista still locks up.
If you uninstall it you can substitute Avast free edition which is just as effective as AVG .
Perhaps some other forum members out there have had a similar problem??
Let deependra know