Windows XP does not provide any method to display folder size in file-explorer, the only method to get the folder-size is to Right-click and select Properties, Folder Size is a nice little free utility to solve this problem, the utility adds a new "Folder Size" column to Windows Explorer displaying size of every folder.
Folder Size works in background calculating and caching the results for faster retrieval later, the service consumes around 5MB RAM and does not slow down your machine.
To install and use Folder Size :
1. Download Folder Size From Official Page.
2. Install it and Open Windows Explorer.
3. Open View menu and click "Choose Details".
4. Check "Folder Size" column and click OK to close the dialog-box.
5. Now, you will be able to see Folder Size column in Explorer Details view.
Source : http://www.megaleecher.net/Free_Utility_To_View_Folder_Size