When i double click, the folder opens in new window. I am using windows vista home basic. Any solution to this overhead ?

The problem could be due to two reasons:
1) Virus infections
2) Folder settings
If it is due to folder settings, you can resolve it very easily. Go to control panel-> Folder Options. I the new opening window, you will see a label Browse Folders. Under that label, you can select the option "Open each folder in the same window".
But, if is due to virus infections, you need to perform a full system scan using Quick Heal antivirus, i have used it in the same situation.

Actually the folder option you suggested above was already selected during the problem. But i don't know how but the problem vanished automatically by restarting computer a couple of times.