When you open any folder inside Windows Explorer (Win+E), it will display the size of individual files contained in that folder but you won’t get to know about the size of various sub-folders that are located inside that folder. The same is true for Windows Explorer available inside XP as well as Vista.
If you find this limitation annoying, what you really need is Folder Size - a free utility that adds a new column in your Windows Explorer to display the size of files as well as folders.
Folder Size installs a Windows service that scans your hard drive in the background, calculates the folder sizes and caches these results, so the results in most cases are almost instantaneous.
To add the Folder Size column in Windows Explorer, right click on the column headers to see a list of columns you can add and choose “Folder Size” and also uncheck the Size column. You may drag the Folder Size column header to where Size used to be. Make this the default view for all folders.
Folder Size is currently available for Windows 2000 and Windows XP but the developer is close to releasing a new version that would work with Windows Vista as well.
See also - Xplorer2, a much better Windows Explorer.