Introducing the fantastic opensource utility - WinDirStat.

Looks like the development has been kickstarted again after having a look at their blog at: http://blog.windirstat.info/

Not to worry, as this is a very basic program and everything just works, and is light weight.

It will scan drives and network drives of your choice, and give you folder size breakdown, including visual representation of the relative size of folders - visually (click on the pic to understand what I mean).

Commercial alternative is TreeSize Professional (the freeware version can not scan network drives). But for what I need - WinDirStat suits my needs exactly!