[quote user="Carnildo"]And yet, the tooltip with the exact size for "windows.old" (which, despite the name, contains not only the previous install of Windows, but also "\Program Files" and "\Documents and Settings") shows up immediately: 1.58 GB. The tooltip for "\Windows", meanwhile, remained at "Larger than 28.5 MB" from when I first noticed it, until I got tired of beating my head against the network and pulled out my digital camera.[/quote]
This is because that information is already cached (probably because you played with it recently).
Try the following:
Right click on the windows folder (hate Microsoft terminology, they're called directories, but thats a WTF for another time) and select properties (this is probably the same Vista as it was in XP), and watch as the OS recursively calculates the size of the subdirectories. When the size stops changing, you know the true size of the folder. Now, it is cached. Try the tooltip again, and be amazed!
Not a WTF, its perfectly sane behaviour (it would be a WTF if popping up a tooltip required traversal of all subfolders)