Unable To Running Low Disc Space?

Like many PC owners, I am running low on disk space. My once seemingly massive hard drive capacity of 185gb is now reduced to around 18gb, which has now prompted me to have a serious clear out.
Problem is, I honestly can't work out where all the used space has gone!
O.K. so, I have trawled through My Documents and held my mouse over all the folders that are likely to contain the most files - like, for instance, My Music, which shows at around 30gb, or, My Pictures, which has a filesize of approx. 20gb.
I have also looked at all the other folders in My Documents, none of which seem to exceed a few dozen, or a few hundred megabytes.
I have also looked at my installed software to see if any programs are swallowing up excessive space. But again, none seem to individually exceed more than a few hundred megabytes, at most.
So, were has all the rest of my missing 100gb gone?
My question is simple.
Is tere any easy and fast way to determine exactly what proportion of the HDD is taken up by what software?
Is there a software program available (preferably freeware) that will scan the drive and tell me exactly whats taking up space, and to what degree?
I have tried a few programs that say they will do exactly this, but after a scan all they seem to display is a pie chart showing whole sections lumped together, such as My Documents - total filesize 40gb etc. But this isn't really that helpful. It needs to be more thorough and broken-down than that.
For all I know, there may be a huge amount of space occupied by some long-since archieved files, of which I am totally oblivious. But, with the average PC holding millions of such files, how could I ever locate the possible cause unless I have software to do it for me?